After searching the web for the best and easiest spray tutorials, I have combined what i learned into Random Hero's Super easy spray tutorial.

You will need:
Adobe Photoshop (or any equivalent such as GIMP)
VTFEdit
An idea :P

Optional:
Custom brushes, fonts, etc.
Renders
Pictures

Instructions:
1. Open Photoshop and create a new file. Make it 256x256 pixels with a transparent background.

2. Create your spray in here. Using Opacity is okay to use as well as any colors you want. Layers are acceptable as well if you decide to use multiple layers. Import a picture, etc. It's all up to you.

3. After you have created your spray, go to File->Save for Web... Select PNG-24 under the drop down menu on the right. Make sure the transparency box is checked and save it to your desktop. Name it whatever you want your spray to be named.

4. Now Open VTFEdit. Go to File-> Import... Open the file you just created in Photoshop. Press OK.

5. Now go to File-> Save As... Save it to the following folder...
C:\Program Files\Steam\steamapps\[THIS IS YOUR STEAM USERNAME]\counter-strike source\cstrike\materials\VGUI\logos
Name it what you named the file you created in Photoshop, but instead of a .png file, it needs to be a .vtf file.
6. Now here is the semi-tricky part... Now open the vmt file that was created when you saved your vtf file. It should look like this...
Code:
"LightmappedGeneric"
{
	"$basetexture" "VGUI/logos/yourspraynamehere"
}
Now change it to this...

Code:
"LightmappedGeneric"
{
"$basetexture" "yourspraynamehere.vtf"
"$translucent" "1"
"$decal" "1"
"$decalscale" "0.250"
}
7. Now go to File->Save.

You have now created your spray. To use it, go into your game and open the options menu. Under the drop-down spray menu you will see your spray. Select it and then join a game.
*Note: If you select the spray while in a server, you will not see a change in your spray until a map change or until you reconnect.

Happy Spray-Making!