Cut avocados in half. Remove seed. Scoop out avacado from the peel, put in a mixing bowl.
Using a fork, mash the avocado. Add the chopped onion, cilantro, lime or lemon, salt and pepper and mash some more. Chili peppers vary individually in their hotness. So, start with a half of one chili pepper and add to the guacamole to your desired degree of hotness. Keep the tomatoes separate until ready to serve.
Remember that much of this is done to taste because of the variability in the fresh ingredients. Start with this recipe and adjust to your taste.
Cover with plastic wrap directly on the surface of the guacamole to prevent oxidation from the air reaching it. Refrigerate until ready.
Just before serving, add the chopped tomato to the guacamole and mix.