Building raised planter boxes is very easy, and usually more cost effective than purchasing them. Follow this easy DIY raised garden box tutorial for an easy design, step-by-step instructions and cost breakdown.
Measure a level line halfway across one of the boards so that you are dividing it into two 4 foot pieces.
Using a skill saw, carefully cut along your level line.
Place the edge of a 4-foot piece of the board even on the edge of an 8-foot board. It doesn't matter if the 4-foot board or the 8-foot board is on the outside. Just keep it the same on both ends so your box is evenly wide all the way down.
Drill pilot holes slightly smaller than your screws into the boards while you are holding them together. It is easiest if someone helps you with this. There should be 3 pilot holes evenly spaced along the edge of the board.
Once the pilot holes are drilled, screw the ends together using the pilot holes.
Repeat this until all the edges are screwed together.
Notes
Follow all safetly guidelines when using any power tools. You are responsible to take necessary safety measures to protect yourself!