Cut the avocado: Use a knife to cut the avocado in half from the top until it stops, meeting up with the pit. Then, rotate the knife around the pit until you come full circle. Twist the two avocado halves apart. This will be easy if the avocado is ripe.
2 ripe avocados (see above notes about ripeness)
Remove the pit: To remove the pit, I think the best way is to cup the avocado with the peel side in your palm and give it a squeeze. We're going to mash up the avocado anyway, so who cares if this doesn't look pretty. If it doesn't release, evaluate whether the fruit is ripe enough to use at all. Then, if you decide to continue, use a spoon to carve it out. There is a method to use a knife on the pit, but I don't think it's the safest method and removing the pit from the knife can be tough on the blade. I'll let you Google that approach. Remove the flesh: With the pit removed, take your spoon again and run it along the inside of the peel to remove all the flesh. Place the flesh in a bowl and mash it with a potato masher or simply with a fork to your desired consistency. I prefer a few chunks rather than a completely smooth texture.
Onion, cilantro, tomato: Chop the onion, cilantro, and tomatoes. Add it to the avocado mash.
3 Tablespoons white onions, 1 Tablespoon torn fresh cilantro leaves, 1/3 cup tomatoes
Lime: Use a lime squeezer or rotate a fork around the inside of the lime to squeeze the juice. Add the juice to the avocado mash.
1/2 teaspoon lime juice
Salt: Add the salt. This is key - honestly, too little can make for a sad guacamole. Remember, you can add more, you can't take it out. So, if you think you need more than I've recommended, just add a bit at a time.
1/4 teaspoon sea salt
Combine: Fold in all the ingredients and serve immediately. If you need to prepare this ahead of time, see my notes below on storage to keep it from turning brown. The lime juice will help, but you will also need to keep it well stored.