how to build a raised garden on a slope | garden guides how to build a raised garden on a slope. hammer in stakes at the 4-foot mark. then take your 4-foot-long 2-by-4 inch board and lay it on the back brick extending out to a stake. place your level on the board and raise the board until the little bubble is in the middle; with a pencil mark the point where the board meets the stake. this is how high your next block or brick wall should be.