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You will need a wooden headboard and footboard. If they are attached, detach them. This project works best if there is either a flat skirt  on the footboard or two-part skirt with an even edge at the top. Once you have the frame and the boards are detached, sand the wood to remove old paint or varnish, if you want to. Measure a vertical center line on the footboard and mark it with a pencil or other marking tool. Use a jigsaw or circular saw to cut the footboard along the center line. The two pieces will become the sides of the bench, while the headboard will be the back of the bench. Along the edges you just cut, drill evenly space holes for dowels. Find dowels that are an appropriate size and then use the drill bit for that size to drill the holes. Measure the height from the floor to the holes that you cut, as well as the distance between the holes, and then make identical holes in the same location on the front of the headboard posts.  The number of holes you drill and where you drill them will depend on the shape and style of the bed. If your bed is of a non-standard shape, you may have to alter how you attach the footboards to the headboard. It may be necessary to attach them at the sides, rather than at the front of the posts. Insert the dowels, applying wood glue in both holes, and attach the footboards to the headboard. It should now be starting to look like a bench! Lay the bench on its back and attach 1x6s (cut to the appropriate length) to the footboards using L brackets and wood screws. Attach these at whatever height works for you and that the footboards allow. Use however many boards are needed to create a wide enough seat. Add a 1x3 board to the bottom of the front seat-board to make a skirt, if you want to. Caulk all the joints and gaps so that the connections are more sturdy. Upright the bench when necessary during this process. Paint the bench (with outdoor paint if you intend to put it outside), if you want to. You may also wish to add seat cushions or upholster the bench.

Summary:
Buy and prepare an old bed frame. Measure and mark the center line on the footboard. Cut the footboard. Create the attachment points on the footboard. Attach the footboards to the headboard. Attach the seat. Caulk all joints. Add finishing touches.