Wear jeans with boots or shoes for this job, not shorts and sandals. For big logs, invest in a decent splitting maul (a long-handled, heavier version of a traditional axe) weighing no less than 3.6 kilograms. This will chop logs along the wood grain without requiring too much strenuous effort on your part.
Use a broad-based log as a chopping block. Place the first log to split on top of it. For rapid results hook a length of chain round the circumference of your log, connected to a bungee cord. This can be adjusted to fit different sizes of log. This holds the log in shape while you work and keeps the pieces together when you move them to your wood stack.
Touch the blade of your maul to the top of the log, around halfway between the edge and the centre and where there is a crack in the grain. With your feet planted squarely on the ground, fix your eyes on the area where you want your first strike to be. Raise the maul straight above your head with both hands. Pull it downwards with bent knees, using your legs to add power. Move yourself round the log, chopping at regular intervals so the log resembles a sliced-up pie from above.
Note: You will need permission from landowners to collect wood for fuel, or a licence from the Forestry Commission for public areas.