An avocado is ripe when it yields to gently pressure, especially at the stem end.
To cut open an avocado slice all the way around the seed. Give it a twist and the two halves should separate fairly easily.
To take out the seed, whack it with a knife so the knife is wedged in the seed, then pull.
Ease a spoon between the skin and the pulp and gently pry the pulp out.
To best prevent any leftover avocado from turning brown, rub it with some lemon or lime juice. Store it tightly sealed with food plastic wrap pressed directly on to the exposed flesh.