Open Roblox Studio
Go to the Avatar section or use the Roblox clothing template
Download the shirt template from Roblox
Open the template in an image editor
Design your shirt on the template
Save the image as a PNG
Upload the shirt image to Roblox
Go to Create or Avatar items
Select Shirts
Choose the uploaded image and submit it
Wait for Roblox to review it
Equip the shirt on your avatar after approval