Decal IDs can be found on the official Roblox Marketplace, where you can find every decal created by a fellow user available for use. Alternatively, you can rely on this page for an updated list ...

Create an image that’s ideally 512x512 pixels and then upload it to ROBLOX via the Asset Manager in ROBLOX Studio. As T-Shirts are just a square decal as opposed to a full shirt, there isn’t ...