The first thing you will want to do is to go to https://tempdecal-conv.pages.dev/
After that, you should be at this page


Now choose the image you want to use for the spray, after adding it, it should start downloading a file by the name of "tempdecal.wad"

Now, you will want to drag the tempdecal.wad to the path below
[C:\Program Files (x86)\Steam\steamapps\common\Half-Life\valve]
After dragging it, it will ask if you want to replace the file.



















Proceed and replace the file.

After doing those two steps, just launch Half-Life and you are good to go! You don't have to go to the spray list in the settings, just go to a multiplayer server and then spray it!

I hope this guide has helped you.