First off, you need to have a program that can open and save .tga files. I recommend Photo Filtre. Other paint programs (other than MS paint) are good as well.
REFER TO SCREENSHOT B
Okay, so open up your paint program. For player VGUIs, create a new picture that has the dimensions "256 X 196". For weapon VGUIs, create a new picture that has the dimensions "256 x 64". Be sure it is EXACTLY those dimensions. Otherwise, your picture will look stretched.
Take a screenshot of your model in HLMV, take a pic from online, or just make one on your own.
REFER TO SCREENSHOT C
Once you're done, save it as a .tga file. Be sure to name it what you want to replace. (e.g. name it arctic if you want to replace the arctic vgui)