The most important consideration in choosing the right avocado is to make sure that they're Hass avocados and that they are rip. Hass avocados (also known as Mexican avocados) are the OG avocados.
How do I know if an avocado is ripe? You'll know that an avocado is ripe if you're able to mush it with your hands slightly (or into oblivion, if you tried really hard).
Cut each avocado in half and remove the pit with the heel of the knife. Use a spoon to scoop the avocado out of its skin and drop these pieces into a large bowl.
I made this short video to illustrate:
Watch the video:
Using a potato masher, heavy spatula, or large fork, mash the avocados into a fine paste.
Chop up the jalapeños and red onions. According to Nate, in cooking, "chopping" food should result in morsels between 1/8-1/4" in diameter.
Next, remove and discard the cilantro stems (to reduce the moisture content), and reduce the remaining cilantro "heads" into a fine mince. Be sure to cut, not crush, the cilantro.
If you have a citrus press or juicer, I highly recommend using fresh-pressed lemon and lime juice. If not, the stuff from a bottle also works well (Chipotle actually uses bottled lemon and lime juice).
This one is self-explanatory. The importance of mashing the avocado first (in the second step) is that, if you mash it with the other ingredients, the onions and jalapeños will get crushed and you'll end up with extremely runny guacamole.
Mix the chopped onions, minced cilantro, crushed salt, lemon and lime juice into your avocado paste.
Enjoy, and don't forget to share the guacamole (and recipe) with others!