You will need:

• A modified rom of some sort
  
• The pak creator (downloadable here[github.com])
  
• An unmodified pak
  
  That's about it.

This is the very simple list on instructions on how to get your mods on the simple launcher.

• Download your modified rom.
  
• Head into the games install directory (C:/Program Files x86/Steam/steamapps/Sega Classics) and go into the data folder. Pick out ANY pak. (e.g "g0021.pak")
  
• Open it in PscdPack
  
• On PscdPack, click "Replace rom" and choose your modified rom.
  
• Rename the modified pak to any 4 numbers you want with a "g" at the start, without the quotes of course.
  
• Put your freshly baked delicious brownies *cough* - modified pak in the data folder in the install directory.
  (C:/Program Files x86/Steam/steamapps/Sega Classics\data)

Now head on to the simple launcher and your freshly baked brownies *cough* - mod should be there.

PLEASE NOTE: I can't help you if anything goes wrong.
I didn't make any of this, I'm just here to share how to do it.
Either way, hope it works for anyone who tried it!