Insert a barrel nut into the hole in each side of the center support slat so the screwdriver slot is facing out. Use a 2 1/2-inch bolt to secure the center support slat to the right-side guard rail and tighten it as much as you can with the Allen key.
Insert one barrel nut into each end of the right-side rail and use two more 2 1/2-inch bolts to attach the headboard and footboard. Thread the bolts through the head and footboards respectively and insert into the corresponding barrel nuts in the right rail. Use the Allen key to tighten these bolts as well, but leave them loose enough to adjust in case later components do not line up as they should.
Insert one narrow support slat on each side of the center support with the dowels lining up to the holes in the right rail. Repeat using the four wide supports; placing two at each end of the bed. Make sure all dowels in the supports line up to the holes in the guard rails.
Attach the left-side rail and thread another 2 1/2-inch bolt into the barrel nut in the center support slat. Insert one barrel nut into each side of the left rail and secure it to the head and footboards by using two more 2 1/2-inch bolts. Tighten with the Allen key, leaving it loose enough to adjust later if necessary.
Use 1 3/4-inch bolts to attach the guard rails to each side of the headboard on the bed. Tighten with the Allen key enough to secure the rails, but leave them loose enough to easily adjust if necessary. Use two more 1 3/4-inch bolts to attach the guardrails to the left and right rails of the bed. Tighten these bolts fully with the Allen key once the rails are lined up so the bolts slide through easily. At this stage, go back and tighten all bolts in the rails, supports, headboard and footboard.