Hi, you will notice that your regular (1.44) disks have 2 holes on the top (one to the left & one to the right). The top right one has a little switch that lets you write protect your disk.
If you want to turn a 1.44 disk into a 720K disk, just put a piece of non-clear tape (make sure it sticks really well as you don't want the tape to get stuck into your disk drive) on the left top hole. Then you can format it as a 720K disk.
Or you can just go to a computer store & buy 720K disks (not all computer stores still carry these kind of disks, though).
Hope this helps,
Loic