@jmcall10
I understand you know how to copy Atari ST games (.st files) to the Atari ST Folder (/home/pi/RetroPie/roms/atarist).

The next step in the doc is to copy a file named tos.img into the bios folder (/home/pi/RetroPie/BIOS).

For some reason, you sound like you suddenly "have no idea what to do" at this step. Why not.

step by step:
1/ On your PC, open your web browser, go to Google (it's a search engine), type "tos.img" in the cell. Click on a random link from the results, read the page that just opened until your eyes see "tos.img" on your screen. Click on that tos.img link to download the file tos.img to your PC.
2/ Do exactly what you did when you copied .st files, except it's no more a .st file, it's a tos.img file now, and you don't copy it into the roms/atarist folder anymore. This time you have to copy this tos.img file into the BIOS folder.