To preview your stickers in-game on any weapon, open WarZone Workbench and then create a new project with the type "Sticker"
You will then need to save your 512x512 sticker image as a transparent png.
You will then need to copy this file to the following location: C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\materials\warzone_itemtest\stickers
You may need to create the warzone_itemtest and stickers folders.
Once you have put your image there, simply select the Sticker in Workbench by going to Edit > Select Material