You squeeze out equal amounts from each tube onto something that can be discarded, like a piece of cardboard, scrap piece of wood, plastic or whatever. If you use a piece of plastic, the hardened epoxy can often be peeled off, and the plastic mixing board can be used multiple times.

Mixing the epoxy also required something disposable. Lollypop sticks work well. Pieces of scrap 1/4 square balsa work well too. I like to use a flat headed screwdriver. The hardened epoxy can be cut off the screwdriver with a knife. Recently I have beed using an old butter knife to mix my epoxy. It works well for spreading the epoxy on stuff.

All in all its mostly a case of using whatever comes to hand. Oh, and one other thing. Make sure you use a mixing board that is bigger than you expect to need. Once you have mixed epoxy a few times, you will get a feel for how much space is needed to mix on.