Having used ice in the past just a few tips...
1. Go slow, one at a time. With 30 seconds between each.
2. Ice will constrict blood vessels and muscles. Essentially the volume of your anus will decrease with each cube you insert.
3. It gets cold suddenly not gradually, and it not something easily undone if too cold.
4. If you have a problem, don't rush to expel the ice. It will be physically difficult and painful to do so. Stop immediately and wait for ice to melt.
5. If struggling to insert, do not force it. Will be hard with constricted hole.
6. Ice will melt and a lose, dilated anus might leak melted water, be aware when wearing any lower clothes.
7. Be sanitary, wash thoroughly before and after play, do not bring hands with fecal particles near mouth or eyes. (can cause pinkeye)
8. Have fun.
I hoped this helps you out. Pm me with anymore questions.