Converting a pair of twin beds into a bunk bed can save more than 20 square feet of floor space in the room. As tempting as it might be to actually attach the two beds one on top of the other, it's easier and safer to build a loft bed you can fit one bed frame beneath. Not only is the final product more stable, but you also can adjust the arrangement later as it suits you.
Prepare the Lumber
Disassemble one bed frame and stow the parts.
Measure the width and length of the remaining bed frame.
Cut two of your beams to match the width of the bed frame. Cut the remaining beams to match the length.
Cut the plywood sheet to match the width and length of the bed frame.
Drill holes in one end of each post, placed on center and 12 inches from the end. Use a drill bit the same diameter as the carriage bolt. Drill a second set of holes in the posts, still on center and 3/4 inches farther from the ends.
Drill two holes in each end of your two longest beams, using the same drill bit you did in Step 5. Stack the holes vertically, 3/4 inches apart, in a line 3 inches from the end.
Sand all faces of your lumber until splinters are no longer a hazard.
Building the Frame
Set the four beams in a rectangular frame with the short beams on the inside of the long beams. All the beams should rest on the 2-inch wide edges.
Screw the rectangular frame together using two wood screws at each point of connection.
Set the plywood sheet on top of the frame, its edges aligned with the edges of the frame. Attach it with one wood screw in each corner and one screw at the midpoint of each long side.
Set one support post in one corner of the frame, aligned so the drilled holes line up. Attach it using two carriage bolts. Repeat with the other posts in the other three corners.
Slide the original twin bed frame in place beneath the new loft frame.
Set the mattresses in place on the twin and loft bed frames.