I wish somebody would build a Square Foot Gardening grid that you can insert into a raised garden bed to mark off squares, but until this is available, if you want to use the Square Foot Gardening method, you will need to devise your own method for marking off squares.
bed gardening is a form of gardening in which the soil is formed in three-to-four-foot-wide (1.0–1.2 m) beds, which can be of any length or shape. The soil is raised above the surrounding soil (approximately six inches to waist-high), is sometimes enclosed by a frame generally made of wood, rock, or concrete blocks, and may be enriched with compost. The vegetable plants are spaced in geometric patterns, much closer together than in conventional row gardening.
A 4x4 foot garden bed, for example, will have 16 planting squares (because there are 16 square feet in a 4x4 bed). A 4x8 foot garden bed will have 32 planting squares.
Numbering starts in the upper left corner and moves left to right, top to bottom. For example, planting square A1 is the upper left-hand square (1st column, 1st row), while planting square C4 is the square in the 3rd column and the 4th row. I have yet to find a commercially available garden bed with an included feature for marking off planting squares.