To add a skin yourself, you must follow these:
1- Go to https://turbowarp.org .
2- Click "File > Load From Your Computer" and open the ".sb3" file of Paper Minecraft.
3- Wait until the project loads.
4- Click "See Inside".
5- Go to the "Steve's Head" sprite.
6- Replace every single costume that uses steve or alex skin (choose only one) to your custom skin.
7- Repeat it for the sprites "Steve", "Steve Arms, "Steve Legs" and "Steve Boots".
8- It's done! If you did everything correctly; save the file to your computer and when you want to play it, you can just load it into turbowarp.