Get some used wooden shipping pallets. Check with grocery stores, hardware stores or any other place that receives large shipments because they will often give away the pallets for free. You can also check your town's classified ads or ask a local recycler.
Take the pallets apart. Most pallets are nailed together, so you will have to pry the boards apart with a crowbar or the claw part of a hammer.
Check the boards carefully for nails and remove any that you can. If there are any nails you can't remove, pound them down.
Choose the boards that are in the best condition. Warped or cracked boards will be hard to work with, and rotted boards won't last long.
Estimate the height of the planter using the width of the boards. All pallets are slightly different, but most pallet boards are approximately 1 inch by 4 inches. The sides of the flower box will be made of boards stacked edge to edge, so it will probably be the height of two or three boards.
Decide how tall you want your flower box to be, and cut eight boards of that length. You will use these boards to make posts that fit in the corners on the inside of the planter. The sides and bottom of the planter will attach to these posts.
Make each post by gluing two lengths of board together. Spread the glue evenly on the face of each board and press them together with a clamp. Leave the glue to cure overnight with the clamps on.
Decide how long and how wide your flower box will be. Cut enough boards for the sides of your planter. For a rectangular flower box that is about 8 inches high, you will need four long boards and four short boards for the sides, plus two long boards and three short boards for the bottom.
Lay two posts with their faces toward you, and lay a long board on top of them with the faces against each other. Place the board so its bottom corner is more or less flush with the corner of the post. The post should be along the short edge of the board.
Drill two pilot holes in the board, then attach the board to the post with two 1 1/2-inch screws. Line up the corners of the other end of the board and attach it in the same manner. Place the next long board on the posts with the long edges of the board flush with the one you've attached, and attach it in the same manner.
Repeat this step with the other two posts and another board. You now have two walls of your planter.
Have a helper hold a short board perpendicular to a finished wall to make a corner, starting from the bottom. Drill the pilot holes and screw the board to the post. Hold the other finished wall to the other end of the short board and attach it in the same manner.
Attach the bottom short board of the fourth wall in the same manner. Add the remaining boards to the third and fourth walls to make a box.
Make the bottom of the flower box by placing a long board with its face against the bottom edges of two posts and screwing it on. Attach the remaining long board to the other two posts in the same manner. Screw the short boards onto the long boards at evenly spaced intervals.
Paint or stain the flower box, if desired. Put your flower pots or potted vegetables into the box.