1.First of all, you need a picture. Open Photoshop or GIMP and create a new file with 256*256px size, then make your spray. 2. When you done with that, download the VTFedit program from here:
3. When you opened it, choose File -> Import, and choose your spray, and the settings window will open. Do not change anything, just press OK. 4. Then put it in the /Steam/steamapps/comonn/GarrysMod/garrysmod/ 5. Click on options then on "Multiplayer" tab. Click on the "Import Spray" button. 6. Enjoy! You can now use that multiplayer spray in GMod with pressing "T" on your keyboard!