Create a Roblox account and join Roblox Premium if you want to upload shirts
Open a shirt template from Roblox’s official clothing template
Edit the template in an image editor like Photoshop, GIMP, or Photopea
Design the shirt on the template using the front, back, sleeves, and torso sections
Save the finished image as a PNG file
Go to Roblox Create
Select Shirts under the clothing section
Upload the PNG file
Name the shirt
Pay the upload fee if required
Wait for Roblox to review and approve the shirt
Put the shirt on your avatar or sell it in the catalog if allowed