Collect two planks of 20-inch 4-by-4 and two planks of 34-inch 4-by-4 for the foot and head pillars, respectively. This is a twin-sized bed.
Measure down 2 inches from one end of each foot pillar and mark a 12-inch-long line in the center of one of the 4-inch faces of the pillar. This is the marking for the mortise that will hold the footboard. On the adjacent side to the footboard mortise mark, measure down 4 inches from the same end and mark a 6-inch-long line in the middle of that face of the pillar. This is the marking for the mortise of the bedrail.
Measure down 2 inches from one end of each head pillar and mark a 12-inch-long line centered on one 4-inch face, then measure down 2 inches from the end of this line and mark another 12-inch-long line centered on the face of the pillar. These are the markings for the mortises that will hold the headboard planks. On the adjacent face of each pillar, mark in 6-inch-long lines for the bedrail mortises that are 18 inches down from the same end you measured down from for the headboard mortises.
Attach a 1-inch router bit to your router and cut a 1-inch-deep and 1-inch wide mortise at each of the lines you marked on the pillar faces. You should have two mortises on each of the foot pillars and three mortises on each of the head pillars.
Collect two planks of 81-inch 2-by-6 lumber for the bedrails, and three planks of 40-inch 2-by-12 lumber for the foot- and headboards.
Measure in 1 inch from each end of all the planks and make a router cut that is 1/2 inch deep and 1 inch wide across the 6-inch face of the bedrails and the 12-inch face of the headboard planks. Make several passes with the router to achieve the 1/2-inch depth so that the cut is accurate. The router cut should be flush with the ends of each plank.
Turn the planks over and repeat the router cuts on the other side of the plank ends, so that there is a 1-inch-thick and 1-inch-long tenon on the end of each of the planks. These tenons will mate with the mortises you cut into the pillars.
Apply wood glue to the tenons on the headboard and footboard planks. Insert the tenons into the mortises on the head and foot pillars, two for the head pillars, one for the foot pillars. It should be a tight fit; if needed, use a rubber mallet to help mate the tenons into the mortises. Ensure that the bedrail mortises are facing the same direction.
Drill three holes 1/2-inch wide and 1-1/2 inches deep through one side of the pillars into the ends of the tenons on each of the headboard and footboard planks.
Cover your dowel pegs in glue. Insert a dowel peg into each of the holes and hammer in with a rubber mallet. Wipe off any excess glue with an old rag.
Glue the square column capitals to the top of each pillar and leave to dry before continuing.
Lay the bedrail planks flat on your work surface. Position a 79-inch plank of 1-by-2 lumber along each of the bedrails, 2 inches down from the top. These are the support cleats for holding the bed slats. Secure the cleats to the bedrails with 1 1/2-inch wood screws driven every 6 to 8 inches along the length of the support cleats.
Apply wood glue to the ends of the bedrails and insert them into the bedrail mortises on the head and foot pillars. This will form the rectangle of the bed frame. The support cleats should be facing inward on the frame. Secure the bedrails with dowel pegs like you did the head- and footboards.
Insert a corner bracket into the inside corner between the pillars and the bedrails and secure with 1 1/2-inch wood screws through the screw holes in the corner brackets. This will give the bed frame added strength.
Space 17 planks of 1/2-by-4-inch lumber, the bed slats, across the bed frame with the ends resting on the support cleats. They should be roughly 1/2 inch apart. Secure the bed slats to the cleats with 1-inch screws through each end of the slats into the support cleat.