Sand all the wood for the bed and stain or paint them as well. Let dry completely before assembling.
Form two planks of 75 inches and two planks of 40-inch 2-by-10 lumber into a rectangle, with the ends of the 75-inch planks butting up against the sides of the 40-inch planks to make the corners. This is a single bed size.
Check with a carpenter's square that the corners are 90 degrees before drilling three evenly spaced pilot holes sized for 3-inch flat-head bolts through the 40-inch planks into the ends of the 75-inch planks. Secure the bed base together with flat-head bolts through the pilot holes.
Insert a third 75-inch plank into the frame in the center and attach it in the same manner as you did the frame.
Place two 50-inch 1-by-5 slats centered across the three 75-inch planks of the bed base, one at either end, flush with the edges of the 40-inch planks. Evenly space 11 slats between the first two.
Secure the slats to the bed base with one 1 5/8-inch screw driven through the slat into each of the 75-inch planks, for a total of three screws per slat.
Pop a screw cap onto the bolt holes on the bed base to finish the solid wood bed.