Measure the mattress size
Decide the bed height and overall dimensions
Gather materials: lumber, plywood, screws, wood glue, sandpaper, finish
Gather tools: saw, drill, measuring tape, square, clamps, sander
Cut the side rails, end rails, center supports, legs, and slats or deck panels
Sand all cut edges and surfaces
Build the outer frame by joining the side rails and end rails
Reinforce the frame with corner brackets or pocket screws
Install center support beams for larger mattress sizes
Attach legs or a base platform under the frame
Add slats across the frame with even spacing or install a plywood deck
Secure all joints tightly with screws and wood glue
Check the frame for square and level
Fill screw holes if desired
Sand the assembled bed again
Apply stain, paint, or sealant
Let the finish cure fully
Place the mattress on the platform bed