Let her watch you brush, and spit.
Let her have fun with it, if you have to. Keep in mind, it's going to take a while for her to learn to spit properly into the sink, without making a mess. Just like it takes a while to learn everything.
Also, if she can understand the concept, you might explain to her that swallowing too much toothpaste will give her an upset tummy. (My oldest daughter wouldn't have followed that reasoning - she's too immediate of a thinker, but my youngest - also two - seems to almost understand, though we're not quite there yet.) Baby toothpaste actually shouldn't give upset tummies if swallowed (if used in a reasonable amount) - that's why it's baby toothpaste - but other toothpaste will, so it's a little white lie I didn't mind perpetuating.