Ok, going to give you all a demo of how I cell shaded the hoodie for my Borderlands Bandit Steve costume. Lets get started. For the garment you are cell shading find one that is a middle ground to the value of the finished piece. In other words don't get one that is to light or dark it will be harder to get the range of values you need. I found a pretty good light brown hoodie on Amazon for $12. I start by inserting a cardboard square into the hoodie to keep it rigid while I draw out my shade lines. I do this in pencil very light. I have screen shots of Steve so I can get it close but then you have to use an artistic eye as well. Once the lines are in I start to add acrylic paint using a mop brush. For the highlight I used Liquitex Unbleached Titanium. It will go on pretty bright buy will fade once the water dries and it soaks into the cloth. This painting process will take multiple layers to get the finished highlight. I have found the best way was to get a good bit of paint in the brush srart in the corner and move along the highlight. Once you get it blocked out take another brush of just water and diffuse the paint by pulling it out from the original brushstroke.
Here is where I start to add the dark value to the hoodie. I use a 12X12 piece of 1/4" MDF scrap that I have lying around as my masking line. I will line up the board with the opposite side of the highlight line I have painted on. I spray the hoodie with Krylon Black primer. The board gives the spray a hard line that will be the ground work for the cell shaded area. I also just randomly hit some spots with the primer to give it more of a grunge appearance.
This is where the fun begins!! I basically trace all of the stitching with an Elmer's Painters Opaque Paint Marker. You really do need to think of how all of these lines are going to lay out before you start because there is no going back. After the stitches are traced I start adding in wrinkle lines here and there as well as the stylized Borderlands flecks of dirt. I am still going back to my reference pic a lot at this point so I can get a sense of the size and weight of the lines. As you can see the lines are not completely sharp but that is ok we are going to go back with acrylic paint on top of this later. The design on the back is also traced with the painters pen.
The lines have been blocked in so I move back to highlights. Instead of using a mop brush this time I have a #2 flat brush. I use this brush along with Liquitex acrylic Parchment to make hard edges on all of the highlights. For larger areas I will use a #6 flat and water to fan the paint out. Again the paint will appear bright at first but will fade some when it dries and soaks into the fabric. Once the highlights have dried completely I start painting in the heavy black lines. I once again use a #2 flat and Mars Black acrylic paint. I repaint the black lines to darken them and make them sharper. Cartoon shading lines are added in the end to reinforce the comic cell shading element.
After allof the black acrylic has dried I mop brush the shadow areas with Liquitex Burnt Umber. By using a mid value hoodie this really sets the shadows back and gives it more of a 3D look and feel.