Hole puncher made them perfect

I did it with an exacto knife. Some of that 5.25 inch floppy material was a little tough.

In some magazines they advertised a special hole punch with some alignment guides to do this, but it seemed expensive.

Most boxes of disks included stickers to recover the existing notch to write protect your disks too...

When I was a kid, I made an alignment tool out of cardboard, and later plastic from old whipped cream containers.

Yup, I made my alignment tool from a floppy disk sleeve. Made it easy to mark the location with a pencil, and then go in with the hole-punch to get it exactly right.

I just used a different disk to measure the depth and a Stanley knife with a fresh blade tip.

Get all nostelgic for those good old days when you messed around with the hardware.