Lay out and spread a bed of mortar
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Snap chalk lines that are a consistent distance ― 1 or 2 inches ― from each side of the blocks.
Use a pencil to mark the openings exactly, then double-check that the openings are the correct width. Use a framing square to check the corners. Dampen the slab.
Mix a batch of mortar and spread a layer about ½ inch thick in between the layout lines.