Let me know if you have any other troubles with it! Modding can be a little tricky to get into here, but it really pays off, once you get the hang of it!
Hullo senpai, I see you're having trouble with your mod shirt. Let me help!
Most importantly, you need to remove your base. The option to do so you can see here this is a link for you to click!. The blank image removes your skin, and thus your arms. BUT! That's where the item I'm sending you comes in!
Head of Adam, the item I sent, can be found in the Accessories tab. Equip it in the base color you like, then in the layers tab, move it all the way to the bottom, under your hair and face and clothes.