@weirdami
How do you load the OS onto an Amiga 500? Same deal.
Actually not quite. I doubt MiniMig supports Amiga Forever encrypted KS ROM images (or does it?). Assuming it doesn't, you first need to start Amiga Forever with the desired KS ROM, and then use application to grab the KS ROM. Then you need to transfer the ripped ROM to the PC side, and finally move it to the flash card.
Then you're ready to go, you just need the game or WB adfs, and add those to the SD, too.