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.
Raised beds lend themselves to the development of complex agriculture systems that utilize many of the principles and methods of permaculture. They can be used effectively to control erosion and recycle and conserve water and nutrients by building them along contour lines on slopes.
Vegetable garden bed construction materials should be chosen carefully. Some concerns exist regarding the use of pressure-treated timber. Pine that was treated using chromated copper arsenate or CCA, a toxic chemical mix for preserving timber that may leach chemicals into the soil which in turn can be drawn up into the plants, is a concern for vegetable growers, where part or all of the plant is eaten. If using timber to raise the garden bed, ensure that it is an untreated hardwood to prevent the risk of chemicals leaching into the soil. A common approach is to use timber sleepers joined with steel rods to hold them together.
Lastly, connect your anchors using a heavy-duty string or twine, creating a grid-like pattern of planting squares. You can see an example of the results of this process in my garden bed here: Square Foot Planting Squares.