1.You need to download skin viewer on:
http://www.minecraftforum.net/topic/274120-minecraft-skin-viewer-10/

2.After you open Minecraft Skin Viewer you let it lunch. once it lunch you chose if you want to type your minecarft user name or drag your skin or someone else skin. You make him look like you want, once you are finished positioning him put background to transparent. To put the background to transparent you press right click, you click on "background" then on "transparent". it should have change the background to little gray and white squares. Now you have to save the image, just do right click and press save image just save it on your desktop.

3.Download gimp 2.6.11 on:
http://www.gimp.org/downloads/

4.You open gimp you will see a tool box and a other window click on "File" after click "open as layers" and chose your picture and open it. Once you did that you have to had a other layer on top of your picture, to do that you go on "Layer" after on "new layer" and set "layer fill type" "Transparency".

5.You can now start painting by using the brush. the perfect size is the second smallest type. To do straight line:
http://www.gimp.org/tutorials/Straight_Line/

6.To do the drawing you need to do the outline of you skin and big detail on your skin (look at the picture up)

7. to color your skin you need to change the color with the color picker tool and after use the bucket fill tool and color an area.

8.Once you are finished click "File" "save as" and save it as a .png

9.enjoy your work!!!

If you need to go back use Ctrl z

don't forget to comment if you need help or subscribe if you liked that tutorial.
I will add a time laps of one of the skin I will draw.