Use magnesium glycinate for all the doses, no nausea and it's better absorbed than magnesium sulfate (epsom).
Also use it every day when not flushing.
The last two doses are not technically necessary, they are only for laxative effect, but in some people it might actually help get some stragglers out.
Drinking a lot liquid or fresh fruit can be as effective as laxative.
Make sure you are properly hydrated every day, and especially the day or two before the flush.
Do the flushes in the daytime so that you can sleep.
If you tolerate apple cider well, it's a very good laxative and source for malic acid. Some people pass stones just from drinking cider.