Design and Preparation
Measure the width and length of the mattress you plan to use for your loft bed.
Cut two beams to match the length of the mattress. Cut the other two to match the width of the mattress, minus 3 inches. For example, a 39-inch-wide twin mattress needs two beams 36 inches long. Cut one plank to match your long beams, and two planks to match your short beams.
Cut the plywood sheet to match the width and length of the mattress. For larger mattresses, such as queen and king sizes, you may need to cut two sheets to half the size.
Measure the ceiling height in the area where you will install the loft bed. Decide how much head room you want above the mattress. For safety, you should have a minimum of 24 inches.
Cut your posts to a length equal to your ceiling height, minus your desired head room. For example, if you had 96 inch ceilings and wanted 36 inches of head room, you would cut your posts to 60 inches long.
Build Mattress Frame
Set the four beams in a rectangle, resting on their 2-inch edges. The ends of the shorter beams should abut the inside faces of the longer beams. This accounts for the "missing" three inches in the length of the short beams.
Screw the frame together using two 4-inch wood screws in each corner. Drive the screws using your power drill with a screwdriver bit, driving them through the faces of the longer beams and into the ends of the shorter beams.
Set the plywood sheet on top of the rectangular frame. Screw it in place using 1 1/2-inch wood screws. Use one wood screw in each corner, and one at the centerline of each edge of the frame. In all cases, drive the screw through the plywood and into the edge of the frame beneath.
Add and Stabilize Posts
Put one post on end in a corner formed by two beams at the underside of the frame. Screw it in place using two 4-inch wood screws, driven through the plywood and into the end of the beam. Reinforce the beam by driving one more 4-inch wood screw through each of the beams and into the sides of the post.
Repeat step one to mount the other three posts in the other three corners of the frame.
Lay the longest plank so its ends overlap the posts on one long side of the frame. It should run parallel to the mattress frame, about halfway between the frame and the open ends of the posts. Screw it in place using two 1 1/2-inch wood screws in each post.
Lay the shorter plank so its ends overlap the posts on one short side of the frame. It should run parallel to the mattress frame, just above the longer plank you already mounted. Screw it in place using two 1 1/2-inch wood screws for each post.
Repeat step four to mount the remaining short plank to the opposite end of the frame.
Maneuver the finished frame into position where you want the loft bed to be. Add your mattress and bed linens. It will be easier if you put the fitted sheet on the mattress before you loft it into position.