Lemme just give a tutorial with pictures (note that the Game.rgss3a file I use is from another game because I already deleted the original one, so the size is different but method remains the same):
1. Extract "Game.rgss3a" in the LonaRPG folder
View attachment 677676
2. Place the extracted files into the LonaRPG folder
View attachment 677700
3. Delete the original "Game.rgss3a" file (or move it somewhere else as a back up)
View attachment 677699
4. To edit the .rb files, open it with notepad (or another text-editing program) and save any changes you did.
View attachment 677682
5. That's it, you can now mod the game.
Hope this helps.
Edit: Removed an extra picture and wanted to say that any changes you make will effect the game immediately, probably better to do it with the game closed. Also, make copies of files before editing in case you make a big oppsie and forgot the original values.
Edit 2: May or may not have accidentally left my name visible, there is no evidence now.