So now that I cant print properly

I'm trying out different things, but I'm wondering if there is a correct (I assume there must be) way to print on both sides of an object, i.e. a plastic keyring. May sound daft but if I do it one side at a time will the other side not be spoilt in some way? Or do I do both sides at once?
I guess I could just do it and find out, but I dont want to ruin the press by making a mistake.
Thanks as ever.