My Suggestion is exporting both Arm Meshes and Importing them into a 3d modeling software called blender(its free), then UV unwrap the model and fix it to your desire. Here’s a quick tutorial assuming that you have imported the model, First Open the UV Editing Tab, then hit A on your keyboard, next hit U, lastly hit ‘Smart UV Project’. The Left Side of the screen will now show the UV map, that you can fix or apply textures. I hope this helped!
If you’re trying to apply the shirt template as a texture then you need to UV Map it to the template itself, since the Roblox made meshes are UV Mapped differently because the humanoid optimizes a lot of stuff.