Halve, pit and slice the avocado. I use an avocado slicer I found on Amazon for $8, and it makes scooping and slicing the avocado guts out an absolute breeze. (Seriously - I try to avoid single-purpose kitchen tools, but this thing is amazing.)
Place the avocado pulp in a large mixing bowl, and juice the lime over top. (This adds a nice acidity, and also prevents the avocado from browning while you work on the other ingredients.)
Add the salt, cumin and cayenne and mash using a potato masher or fork. You don't want this to be perfectly smooth, so make sure there are some chunks left in for texture.
Add the onion, tomatoes, garlic, cilantro and jalapeno and stir to combine. I add about 3/4 of the onion in at once, mix it together, and then taste it and decide if it needs more. Too much onion can really overpower the guac, so I add more, mix it, and taste as I go until I hit the right balance. Same goes for cilantro.