Make sure they are on the loosest setting; take it to the shop and have them check if you are not sure.

Then, take the bike to a grassy area. Practice clipping in and out on both sides (not just one) cause if you only do one something will happen and you will fall the other way.

Or, put your bike on a trainer first, and practice clipping in and out and in and out until you are used to it.

Now, your old ways may still be dominant, and you may still fall over at least one more time; but generally it does not kill you; you get a few bruises and some reinforcement that you don't want to do that again and you will be fine. It is like anything; it takes a little practice.