Use Roblox's size guide for each template part. Now it's time to create an image for the shirt. Open the template in any image editor; some free options recommended by the developers include GIMP ...

Roblox is a large-scale multiplayer ... Choose the appropriate shirt template from the drop-down menu. Adjust the shirt's position and size using the editing tools provided. Click on the "Save ...

Gamers can customize their avatar by creating clothing, like shirts and ... For each template, notice that the parts are "folded up" and wrapped around a Roblox character's body, arms, and ...

so you'll need to design a t-shirt in an image and then upload it to Roblox. From there, just select it to wear and you're sorted! You'll be best off having a couple of templates for this!