This is not rocket science as long as you have the proportions right. Feel free to experiment. I love tomato in my guac but if I make enough that I know I'll end up refrigerating it overnight I don't add tomato because it ends up tasting funny the next day.
2 ripe avocados, peeled and pitted
1/2 small onion, finely chopped
juice of 1 lime
1 jalapeno, seeded and chopped
1 handful cilantro, chopped
1 tomato, seeded and chopped (optional)
Pinch or two of salt
1 clove garlic, minced
Dash or two of ground cumin (optional)
Smash avocados and add remaining ingredients. Mix. Eat.