For the uninitiated here goes:
Place a garlic clove on a chopping board. Use a large chef’s knife (flat side) to smash the garlic clove a couple of times making the removal of skin very easy or simply peel the clove in your usual way. Place a small amount of salt beside the garlic clove and using the blunt side of a table knife gradually scrape the clove with the salt, a little at a time, until it becomes a paste. The salt acts as an abrasion, absorbs the oil released in the process and helps to preserve flavour. Once crushed the garlic begins to change colour and become very strong and acrid.
Peel an onion and trim the root end but do not cut if off – it needs to stay in place to hold the onion together during the chopping process. Cut the onion in half from the stem end to the root and place one half, cut side down, on a chopping board. Using your fingers on one hand firmly hold and press down on the onion. With your other hand make horizontal incisions across the whole width of the onion, evenly spaced apart, but stop just before the root end after each incision. Take the knife and cut the onion at right angles to the root and across the onion in two places, one just above the other. Now slice the onion parallel to the root to produce a perfect dice. Repeat with the other onion half.