The Apple Disk II drive is capable of reading only one side of a floppy disc. To use the other side of a Double Side (DS) disk, you need to flip it like a vinyl record. However, only flipping won’t do the trick.
All 5.25 floppy disks have a cutout on the right-hand side which tells the drive they can be written on. However, if you flip it, there will be no notch, and the drive will not be able to write on that side.
In the past, we had a tool called the Floppy Disk Notcher. But for inexplicable reasons, it became too expensive, and in the end, all you need to do is cut the disk in the right spot.
I use an X-ACTO knife, but if you’re careful, you can use a good pair of scissors as well. To tell where to cut, I use masking tape (see image below) and trace the line by flipping another disk on top of the one I’m cutting.