Mashing garlic is easy with a mortar and pestle, but if you don’t have one (or don’t want to dirty one), just use your chef’s knife. Here’s how:
Trim off the ends of the garlic cloves and slice the cloves in half lengthwise. Turn the cloves flat side down on the cutting board. Lay the side of the blade of a chef´s knife on each clove and smash down with the heel of your palm. Sprinkle the garlic with kosher salt and chop coarsely. Pile the chopped garlic on one side of the board. Tilt the knife at a 30-degree angle to the board and drag it over the garlic, scraping it across the surface of the board. Pile up the garlic again, sprinkle lightly with salt, and scrape again. Repeat once or twice until the garlic is a smooth paste.