|Normally I use a slab of glass as a palette.|
For this tutorial, I'm using white paper for a clean background.
Purchased colours are great for consistency. If you're painting a large army, it's best if you don't have to mix the colour for every model, because it won't be perfect every time. If you do need a mixed paint for a large uniform army, I'd suggest mixing a bunch up in an empty paint pot. (Which you can buy at hobby stores, re-use an old one, or use travel kit bottles from dollar stores.)
When you're doing the actual mixing, there's a couple ways I'll tackle it: