Measure your space to determine if the garden will fit. If not, adjust the measurements to fit your deck.
Measure 4 inches from one end of the 6-foot lengths of 2-by-4 inch wooden boards. Make a mark at the measurement and cut the 6-foot long 2-by-4 pieces on the mark.
Place the sheet of Medium Density Fiberboard flat on the work surface. Place one 4-foot piece of 2-by-4 on one of the shorter ends standing on its side. Secure with clamps to keep in place. Flip the piece over and drill pilot holes every 6 inches through the MDF into the 2-by-4. Attach with wood screws.
Place one of the 6-foot boards on top of one of the longer sides of the plywood so the short end of the 2-by-4 is butted against the 2-by-4 already attached. Hold in place with clamps. Flip the board over and drill pilot holes through the MDF into the 2-by-4. Attach the two pieces with wood screws.
Repeat for the other two 2-by-4 pieces to make a shallow box. Sand the box and paint or stain as desired.
Place the larger boulders into the garden in a random fashion. No matter what size garden you have, you will want boulders spaced a good distance from one another.
Fill the box with sand until the sand is about one inch below the top of the frame. Spread the sand around the boulders and level.
Rake the sand around the boulders and in the box to make a pattern.