You are going about this the right way. Yes, practice first!
The first thing I would investigate is whether or not you mixed too much resin. Yes, resin will get warm, but if you mix too much at once, it will get very warm. Ask the manufacturer what their recommended minimum and maximum mixing amounts are.
If you are within those volume numbers, then try a different way to seal your flowers. We have a few ways on how to do that here: https://resinobsession.com/tag/drying-flowers