Open Roblox and go to the Create page
Make sure you have Roblox Premium
Download a shirt template from Roblox
Open the template in an image editor
Design your shirt on the template
Save the finished image as a PNG
Go to the Create page and select Shirts
Upload your shirt image
Name your shirt
Pay the upload fee if required
Wait for Roblox to review and approve it
Wear or sell the shirt after it is approved