Ok so you want to learn how to make a skin? You've come to the right place.
You will need:
To start us off exit Steam completely.
Next start up GCFScape and go to File>Open it will take you right to your Steamapps folder, open up source materials.gcf
After you open source materials.gcf you will see these files
Browse to hl2>materials>models>humans>male and Right click Group01 and extract it to somewhere, I suggest making a folder called C:\My Skin and save all your skins there to keep organized, so extract Group01 to C:\My Skin
Go to C:\My Skin and open up Group01>citizen_sheet.vtf with Photoshop (make sure you have installed the vtf plugin correctly)
Now for the fun part: skinning
First things first, go to Image>Image Size and change it to 2048x2048
Now take out your Polygonal Lasso tool, and cut out both of the shirts. Make sure you get just the shirts and nothing else (like the small under shirt round triangle things).
After cutting out the shirts go to Image>Adjustments>Desaturate while the shirts still selected create a new layer, fill in the layer with whatever color you want, and then right above Layer 1 there will a box that says Normal, change it to Overlay
You will notice the color takes form to the shirt and you have recolorized it.
Move onto the pants now, cut them out like you did with the shirt, careful not to cut out anything else.
I usually don't edit the pants too much, just make them brighter, or put a pattern over them
Follow these instructions
We're almost done now! Just go back and look at everything and make changes to what you want and get things how you like it. After you have done that save it as citizen_sheet.vtf in your C:\My Skin\Group01 folder, when it asks to overwrite the old citizen_sheet.vtf say Yes.
Congratulations! You've just completed a skin, now time to go test it in game.
Browse to your Steam\steamapps\garrysmod\garrysmod\materials\models\humans\male now open up C:\My Skin and copy paste "Group01" into "male". The skin will now be put in game and ready to use.
Thanks for reading, I hope I helped someone.