Okay, let's start with the PC side of things and ADF Opus 1.2 ...
Load it up, select New from the File menu.
Leave every setting alone, apart from the very last - make sure Open After Creation is selected as shown in the picture ...
Click Create and a new box will open up. This is your ADF file - you can simply drag and drop your stuff in there just like you do in Windows ...
Now click on the Windows Directory window to bring it to the front. Using the Action menu, keep selecting Up One Level until you see all the drives listed (C: D: E: etc. as shown).
Navigate to where your MOD files and PT1210 is stored just as you would if you were using Windows. Now drag your files into the NEW ADF window. (might help if you make the main Opus 1.2 window full screen first, giving you more room to move the others around so you can see better).
In the above example , I dragged and dropped the 3 files from the Windows Directory folder into the NEW ADF one. The program won't let you add more than 880kb because that is the standard Amiga disk size (plus it was pre-selected when you first ran the software).
When you are finished adding your files, remember to select Close from the File menu or your work won't be saved.
Now copy that nice new ADF file over onto your USB stick, go plug it into your Amiga Gotek, select it and enjoy.
If your A600 has a HD or CF/SD card HD then simply copy from the mounted ADF file onto the HD just as you did with the Opus 1.2 software - drag and drop!