Make sure that you are a member of Roblox Builders Club.
If you aren’t a paying member club subscription, you aren’t allow to upload any shirt template.
Right-click these images and select Save Image As… to save the templates to your computer.
At this step you should use your favorite photo-editing program like Paint, Photoshop, Lighroom, GIMP, Paint.net or any other.
That all! Now you have to wait till your design pass moderation and it will be available to wear or sell!
Feel free to copy any template you like but edit it before upload to Roblox.