Make sure the avocado is ripe and ready to slice.
If the avocado is rock hard, it will need a few days to ripen, but if an avocado yields slightly to gentle pressure, it is ripe enough to slice. Avocados which dent slightly when squeezed are too ripe to slice, but are suitable for mashing. However, avocados which feel hollow in places or dent too easily are overripe and will have darkened and spoiled inside.
RUN the knife around the avocado, lengthwise, slitting it the whole way around.
TWIST the two halves in separate directions.
SEPARATE the two halves of the avocado.