It's possible to save big bucks on minis by purchasing them on Ebay, but usually you're going to have to work on them some to get value back out of them. Don't just paint over someone else's crappy base coats and bad prime job!
It's also useful for fixing a model you don't care for, or perhaps updating a favored model for a new army. Anyway, put the models in a glass jar, fill it with the cleaner, and put the lid on. Keep your mug away from the fumes and wash your hands right after... and yes, I speak from experience - it burns a bit if you get it in your eye.
I usually do this when my wife isn't around because inevitably you'll have to clean the sink. Keep the water going and catch the paint in a strainer; don't just let it go down the drain.
Use an old toothbrush and some elbow grease - remove as much as you can.
Personally I've always used Purple Power. It'll work for both metal and plastic... and it won't melt anything if you forget the models are in the drink! I've left models for a long, long time before, but it didn't hurt anything.