Create your shirt design in an image editor
Use the correct Roblox shirt template
Make sure the image size matches the template
Save the file as a PNG
Go to the Roblox Create page
Open the Shirts section
Click Upload Asset or Choose File
Select your shirt image
Enter a name for the shirt
Pay the upload fee if required
Submit the shirt for upload
Wait for Roblox to approve it
Equip or sell the shirt after it is approved