Scoop some ice cream into a bowl. How much you need depends on the size of your molds and the number that you're making. For my small-ish molds, 3 scoops does it.
Let the ice cream soften a bit. You can microwave it for a few seconds to hasten the process.
Stir in some mix-ins.
Spoon the mixture into the molds.
Put the lids on the molds. Freeze.
When ready to serve, I usually run the bottom of the mold under warm water for a few seconds, then they pull right out.