I have struggled with this same issue myself. I am a watercolor painter, so having clean line work is essential before beginning on the painting. Being an extremely messy sketcher, erasing lines over and over again, drawing directly on my watercolor paper was never an option. I found that I tore up the fibers of the paper which made painting on it a nightmare. Also, I have never cared much for leaving graphite on the paper and then painting with watercolors on top. If you're not careful, this can make the colors look muddy.
So when faced with this problem initially, I went through all the standard solutions. I tried graphite tracing paper. This was messy and I could never quite get my sketch to look the same way on the final paper. It was also quite time consuming.
I have also tried a light box and a projector to get my lines on the paper, but again you're leaving graphite directly on the paper which I'm not crazy about. Some artists do very well with this technique, but it has never been for me.
After trying and discarding all techniques to get my line work onto my final watercolor paper, I became very frustrated! Then it occurred to me that I have knowledge of Photoshop and know how to create nice clean line work in Photoshop! So I started creating my line work in Photoshop and then I print my lines very lightly onto my final watercolor paper. I run my watercolor paper right through the printer. It works fabulously! I then ink over the printed lines and paint from there. I have been using this technique for years and it's the cleanest, best technique that I have found.
Also, the added bonus is that you have a super clean digital black and white line version of your artwork. If you sell your work, there are many different uses for this black and white version. An obvious use of it is to sell coloring pages of your work. You can sell them as printable files or as actual pages that you mail. And if you license your work, and have a nice selection of black and white line versions of your artwork, consider approaching a rubber stamp company. They use these black and white versions to create their rubber stamps.
I also want to add that my art is stylized. So I have a hard line in all my art. This technique of printing the lines on your paper works best for that type of style. If you don't have hard lines in your work, you may end up seeing the printed lines in your final painting and there's no way to get rid of them if you're using watercolors. But this is also a nice technique for acrylic artists or any other medium that would hide these lines that you're printing out.
Ok, so that's all great, but you're probably asking...how do you do it! Well, I'll try my best to show you how. Let me start by saying that there are probably much better tutorials out there for what I'm going to show you with people that have much more knowledge than I have. But I've been doing it this way for awhile and it works for me, so I figured you all might enjoy seeing how I do it! To be able to do this tutorial, I think at least intermediate knowledge of Photoshop would be helpful.
Step One: Create a Sketch
You have to have a sketch to start this whole process. I prefer the old paper and pencil approach. I have just never gotten used to sketching with a stylus in Photoshop directly. But if you like to sketch in Photoshop, by all means go ahead. You just need a sketch of any variety to move on to the next step.
For this tutorial, I will be using a mermaid sketch that I finished awhile back:
You can see that I did try and make the sketch as clean as possible. It is important to know exactly where you want all your lines to go. It will save you a lot of headache later on. Trust me! But there are also some smudges and some lines in there that won't make it to the final version. But that's ok. We will clean that all up. Just know in your mind what lines you want and which ones you don't.
Step Two: Setting Up Your Photoshop File
You need to scan in your sketch first. I recommend scanning in at 300 dpi to create a nice high resolution version of your sketch.
Then, make your Photoshop file where you will create your line work. All of my line work files are 8 inches by 10 inches and 600 dpi. I like to go larger with the dpi if possible and if your computer can handle it. At the very minimum, you need to have 300 dpi. But I like to go larger so that I have more options.
Next, set up your layers so that your sketch is the 2nd layer from the bottom. Your bottommost layer needs to be plain white. Here is a photo of what my workspace looks like when I first get started.
Also, I would recommend lowering the opacity of the sketch layer by 50% so that you can still see the sketch easily, but you will be able to focus on the line work you're creating in Photoshop better.
Step Three: Use the Pen Tool to Create Line Work
Next, lock the background layer and your sketch layer. You want your line work to be separate from these layers. It's best to lock to them so you have no chance of combining the layers. It's a nightmare to try and separate them, so be careful with that part!
And then create a new layer where your new line work will be. Select the Pen Tool and select Paths at the top. If you've never worked with paths, I might recommend searching for a step by step tutorial on how to use paths. Honestly, it just takes a lot of practice to get used to how to use them. It's still challenging for me at this point sometimes.
Basically, you create anchor points by clicking on different points of your lines. Then little handle bars pop up that you can adjust to create the curve in between two anchor points.
At this point, I usually zoom into my piece between 100-300% so I can really see what I'm doing. Right now, I'm working on a curve in her hair. I left clicked my first point and then left clicked and dragged slightly to create my second anchor point. When you left click and drag it creates the handle bars. This picture shows where I initially put the two points and the line in between:
But you can see that the line isn't quite where I want it, so I need to use the handle bars to adjust it. You hold down control and left click+drag to move the handle bars around. This is really the part that takes practice. You need to learn where to place your points and how to adjust the lines. You can see in this next photo that I've moved the line into position of where I want it:
At this point, you have the path, but you need the actual black line. So you need to stroke your path. What you do is select the brush type and thickness you want to use. I use a regular round brush size 5. But you can use whatever you'd like. Then click the Paths tab in your Layer window. Then at the bottom of the window, there's a button that says "Stroke Path with Brush." Hit that button and it will stroke your line.
After that, I usually delete my path, because it just gets in your way. That was a simple line that only had two anchor points. Sometimes you have to create a line that has multiple anchor points. I suppose technically, you could create the entire line work in one path, but I prefer breaking it up into lines. That just makes it easier for me. You can see with this next line, I'm working on some of the scroll work. Extreme curves like this scroll work, are often the most difficult to get right. You sometimes get these pointy areas that area really annoying. But with practice, it becomes a lot easier. I created this scrollwork with 9 anchor points.
Then stroke it and delete the path.
Here is a useful thing to keep in mind:
Alt click an anchor point to create a "hard turn" in the curve. I'm sure there's a technical term for this, but it's just not coming to me right now. You can see in this next picture, that I want the curve to kind of stop and start again at this junction:
So I simply Alt+Left Click that anchor point and it makes a nice hard turn in the curve.
Next, I will jump ahead to the completed piece. I will use another one of my pieces that I finished as this mermaid one is not done yet. =)
Here is the completed line work of a witch piece I did awhile ago called Ashlyn:
Step Four: Preparing Your Image for Printing
Once you get to this point, you simply hide or delete your sketch layer and then you're left with a nice clean version of your artwork!
Next, you want to print this version on your final watercolor paper. So figure out what size paper you're going to put in your printer. I recommend not printing borderless. I would leave space all the way around your painting so that you can tape off your paper. For instance, this linework measures 8x10", but I would put a piece of paper that's 8.5x11" into my printer. Here's what my file looks like when it's ready for printing:
You can see that my paper measures 8.5x11", but the image is only 8x10." I then add a line around the image so that when I'm taping/painting, I know exactly where the end of the painting should be and I don't end up with any blank spaces that should be colored. I usually tape down my pieces and use the line as a guide. You may also notice that I decreased the opacity of the lines. It's important to get the lines as light as possible so that you don't have any chances of them showing through on the final version if you're using a medium like watercolor.
Step Five: Print, Tape, Ink and Paint!
Next, you just load up your watercolor paper in your printer and print it out. You need to make sure that your printer will allow for a thicker paper like watercolor paper. Some do, some don't. Make sure you check before you try to feed through a thick paper! I recommend printing with a photo setting so that your lines are nice and crisp. If you print with a lower setting, sometimes the lines get distorted. You don't want it to be too light so you don't know where to ink.
After this, I tape down my painting and start inking! I use a waterproof brown ink with a dip pen. Here's a picture of what Ashlyn looks like midway through the inking process:
You can see the very faint lines that I printed off and then the brown lines are what I have inked by hand on the painting. Then, you just continue inking and painting until it's complete!
Now, we're finished! I hope you enjoyed that step by step process of how to create clean line work in Photoshop. I really hope that it will help you in some way in your painting process! =)