Raised garden beds can be constructed with landscape timbers, wood slats, concrete block, rocks or other supporting materials and are easy to build almost anywhere--even over a section of lawn. No tilling is required to remove existing grass or weeds before planting.
Lay out courses of newspapers, several sheets thick, over the grassy areas where the raised bed is going to go. Gently spray the sheets with a water hose to weigh them down and allow them to sit flatter. If necessary, weigh them down with rocks, bricks or timbers to keep them from blowing away. The newspapers will form a barrier to help keep grass and weeds from growing up through the bed.
Lay one course of landscape timbers around the outside edge of the newspaper, cutting the timbers to fit the dimensions that you want the raised bed to be. Butt them up together tightly and be sure that all of the timbers are covering the edges of the newspaper, without any grass showing inside.
Lay the second course of landscape timbers on top of the first one, but alternate the ends of the timbers that meet at the corners. For example, if the bottom course has a lengthwise-spanning timber butting up next to the end of a the width-spanning timber, the second course should be in the opposite direction; this will form a sturdier corner.
Drill holes completely through the timbers 2 inches from the ends and again approximately 3 feet apart down the length of the timbers. Drive lengths of rebar down the center of the holes, anchoring the timbers to the ground and each other leaving as much rebar as necessary sticking up out of the timbers to complete the desired height.
Continue laying courses of timbers--alternating the corners--by drilling holes where appropriate to accommodate the rebar sticking up, until the raised bed is the desired height. Do not extend beyond the height of the rebar.
Pound the ends of the rebar down with a small sledgehammer to form a blunt end. Drill two holes in each landscape timber through the top two rows. Drive the landscape spikes in through the holes until the head is flush with the top timber. This will lock the top timbers in place and prevent them from being kicked up.
Fill the raised bed with topsoil and add compost to the top 4 inches of soil to help enrich the soil and prepare it for planting.