You will need gimp (I use Gimp 2.6) absolutely, But if Gimp confuses you, you can use paint or paint.net to draw the skin and use Gimp to shade it.

Once you've installed the programs(GIMP And whatever progrms you want to use), draw the skin and save it. I recommend downloading the skin template from Guncraft and resizing it to 128 by 128.
This is my minecraft skin I made, shaded, and ported to Guncraft

Just copy the template, open the template copy, resize it to 128x128, fill in the blanks and save.
EASY RIGHT, THIS IS THE HARDEST PART OF SKIN MAKING

Yes, you heard me correctly, shading. It is not hard, if you have GIMP. First, open your skin if it's not already opened. next use the Dodge/Burn tool.
This is where it's located in GIMP 2.6.










Set the scale to .01. Then use the Dodge tool to lighten things up. If it gets too bright, use the Burn tool to darken it. Then save it.
The dodge is on the left, and the burn is on the right.

Make a copy of your skin and open the copy. Next make everything you want to be team colored white. and everything you want unchanged black. Then save it.
This is what the stencil for the skin I showed you earlier looks like.

The easiest part. Run Guncraft, go into the foundry, and click "Custom Skins". Select the skin file (The Skin You Made) and your stencil (the Stencil You Made). Edit as needed. Click Upload and Enjoy!!!!!!!!!!!!!!!!!!!!!!!

Thank you for reading. and Thank [GSR] The Not Bruce [GWC] for teaching me how to shade.