Instructions
Log into your "Roblox" account and go to your character customization page. Click on "T-Shirts" and the "Create" option. The shirt creation template will appear.
Download this file to your computer's Desktop, and then open it using a graphical editing program such as Microsoft Paint.
Paint your jersey using the program's tools. The top section of the template is what will display on your character's torso, with the center square being the front of the jersey. The two bottom sections are what will appear on your character's left and right shoulders and arms.
Save the changes you have made to the jersey file once you are finished. Return to your character's customization page.
Click the "Browse" button in the "T-Shirts" section and choose your jersey file from your computer. Click "Create Shirt" to upload the jersey to your account. It should now appear as an option on your character customization page.