Size the trench so there's enough room for the retaining wall block and at least 8 in. of space behind it. Excavate deep enough to completely bury at least one full course, including space for 6 to 8 ...

My daughters grew up playing a video game called Tetris, and we recently used the skills they learned from that game to build a wall. The Wikipedia definition of the game is this: “In Tetris, players ...