I used a flat needle file sitting in a vise. Holding the ring with gap squeezed together around file and move ring. Only one direction, from outside to inside.
If you have modern piston with very thin oil rings, the rails need to get the inner end corner rounded to make it easier to compress them.
The sharp corners of the rails can hook into the expander and not compress enough when bore need to go on. The rail can get pinched and you need to order new. If not discover that forcing the bore on you will harm the bore.