for outlines I use GIMP (if you can download it my instructions will help)
-set up a picture like in LadyM's tutorial (lines above a white layer)
-change the opacity of the original lines to REALLY low (~20-30%) so you can barely see them
-make a transparent layer under the lines
-make a REALLY small brush (1-3 px dm and soft) and use airbrush; brush size may be optional depending)
-this makes the brush take a LONG TIME to put down color (constant back and forth motion

), but it allows you to gradually get your lines right, rather than laying down a hard lines and undoing constantly
-go over your lines (duh); take your time (tweaking is allowed of course)
-once you have them the way you want, just delete the original