VC Victa,
1. You can use plastic sheet to rap around the piston and then slide the ring down to the bottom groove first.
2. You put the ring leg into the first groove and try to walk it down to the bottom grove (you need to be careful not to break the ring).
3. You can buy a ring expander.
The first option is the cheapest and easiest way and the second option has risks and the third is the professional way.
