Once you are done, just upload it to Roblox and you're sorted - just make sure the image is 585 pixels wide and 559 pixels tall else you will run into problems as it uploads.

How to Make T-Shirts in ROBLOX T-Shirts are the easiest clothing to make in ROBLOX. Create an image that’s ideally 512x512 pixels and then upload it to ROBLOX via the Asset Manager in ROBLOX Studio.