1. Make your decal in whatever you are using, make sure it has
transparent background, not white, save it as .png.
2. Open VTFEdit, click on "File => Import". Click Ok.
3. Save it to "[your Steam folder]\steamapps\common\Team Fortress 2\tf\materials\decal_example"
4. Then click on "Tools => Generate VMT File", make sure "Translucent" is checked on and save it to the same folder.
Here it is!