Create a Roblox account and log in
Get Roblox Premium
Open Roblox Studio or use the Roblox website
Download the official shirt template
Edit the template in an image editor
Design the shirt on the template
Save the image as a PNG
Go to the Create section on Roblox
Select Shirts
Upload the PNG file
Enter a name for the shirt
Pay the upload fee if required
Wait for Roblox to approve the shirt
Wear or sell the shirt after approval