1. go to the picture you want to overlay (such as my cosmetics or just pictures)
2. do the select all keybind on your computer or manually select everything
3. copy the selected area (everything)
4. go to the Minecraft skin you want to overlay stuff on or you want to add cosmetics to
5. paste in the stuff you want to overlay or the cosmetics
6. align and/or rotate the thing you want to overlay on the skin to the right place
7. save
8. you're done!
nice job!

if you have any questions, I will try to answer them as best I can.