Step 1: Passionlip deploys one rocket fist. Step 2: Lip very carefully pulls the shirt down over her body with one hand. Step 3: Lip pushes the hand-less arm through a sleeve. Step 4: Re-attach rocket fist. Step 5: Deploy other rocket fist. Step 6: Push the hand-less arm through the sleeve. Step 7: Re-attach rocket fist. Step 8: Very carefully adjust shirt as necessary.
The part where her humerus is doesn't detach like her rocket arms does, that is where the "how" comes.
... Huh. Upon looking again, you're right - part of the arm that remains is still too wide.
Either the sleeves are a lot stretchier than they look, or someone is using some sort of magecraft to repair the shirt after it inevitably tears in the process of trying to be worn. In the former case, the previously-mentioned steps might still work.
