Holding the avocado in one hand, slice through the stem end until you hit the pit. Holding the knife relatively still, twist the avocado around until the knife blade has cut all the way around the avocado and ended back where it started.
Put the knife down and grab the avocado by each half gently twisting it until it comes apart. One half will be empty and one half will have the pit. Put the half with the pit in the palm of one hand and grab the knife with the other. Gently but firmly cut into the pit with the knife. Twist the avocado in a different direction from the knife with the pit stuck to it. They will come apart and you will have two halves without a pit.
Slice two or three times lengthwise and two or three times crosswise and you will have cubes of avocado. Just take a spoon and scoop them out.