Okay, super cheezy title.

Here's a recipe for homebrew cels that has really worked
for me.

Step 1-You've got to start with the basics, namely a genga!
Do a few rough drafts the depict shading and highlights.
Sorry to steal your Genga pic Chad, but it's a great 
example!

Step 2-Finalize all your shading and highlights. Using
red and blue pencil to differentiate highlights and shading
helps nicely.
Almost all my gengas are extremely sketchy. For my step 2
drawing, I like to trace over the original, only leaving
the black lines I'd want to see on the cel.

Step 3-Scan the finalized line drawing from 2, then print 
the drawing out on a transparency. I have a nice laser 
laser printer so my lines come out VERY crisp and clear.

Step 4-Take some colored pencils or pens and do a color rough draft of your cel. Mix your paints after you've 
printed them out. Be warned! If you use acrylics, the paint
will dry very quickly!  Squeeze the paint out of the tube
as you use it. 

Step 5-Using a soft brush, paint your cel =)

G'luck!