when you make a spray with a program (ex photoshop), you goto file>new. and it will ask you the width and length of the image, you make it divisible by 16 (if you dont know what that means ill tell you, but unless your 5 you have no excuse) this means like, 16, 32, 48 etc, like 16x2=32, 16x3=48. GET IT?! make the length and width divisible by 16, when you get that, itll give you a blank white image you can customize to w/e you want, get decal converter or wally and make it a wad file, save it to (default):
C:/Program Files/Steam/SteamApps/your@e-mail.com/counter-strike/cstrike
then after the pldecal.wad is saved into your cstrike folder, your....not done, rename it to: tempdecal.wad or tempdecal, restart steam, join a server that allows custom sprays (it never says just try a few before you see it work.) good luck, if you dont get shame on you, thats step by step dude!!!