The how to, you ask? I start with a good, shallow sauce pan (ahh...all-clad) that will both work on the stovetop and in the oven. With the oven preheating, I coat the pan in olive oil, saute onion, garlic, then whatever veggies I'm using. Today I started the zucchini first then threw the spinach in right before the egg. Be careful with tomato, the liquid can cause your eggs to not set properly. I typically just slice tomatoes and float them on top when the egg is almost set. I whipped up four eggs, a splash of milk, salt, pepper and a good tablespoon or so of finely chopped oregano and rosemary.
Drop the pan to medium heat and dump the eggs over the veggies and allow to cook most of the way. Don't mess with it too much!
You can run a knife around the edge to loosen but it's best to just leave it alone. When it's close to set - especially around the edges - I pop the whole pan into the oven (350, 400 ish) to finish...about 10 min but each time seems different. Just watch for the eggs to set. If I'm really feeling ambitious (I was today) I quickly finish it off with a visit to the broiler to crust up the top a bit.
See, there's a reason I don't really write recipes here - I don't use them. I peek at some to get a basic idea of what to do, and then experiment.
Once you have the basic concept down, it's so easy to be creative with frittatas. And they make a fantastic holiday brunch.