Measure the length and width of the mattress for your toddler bed with a tape measure. This gives you the dimensions of your platform bed frame.
Cut two pieces of 1-inch-thick-by-24-inch-wide hardwood boards to the length needed with a miter saw or table saw. These boards are the sideboards of the platform bed.
Cut two pieces of 1-inch-thick by 24-inch-wide hardwood boards to a width 2 inches shorter than needed. This accounts for the thickness of the sideboards. These two boards are the headboard and the footboard of the platform bed.
Drill four evenly spaced 1/4-inch holes across the width of each sideboard approximately 1/2 inch down each end of the boards.
Position the sideboards on their 1-inch sides and then position the headboard and footboard on their 1-inch sides. Line the shorter boards up at the ends of the sideboards.
Drive 1/4-inch-diameter by 2-inch-long wood screws through the holes in the sideboards and into the headboard and footboard. Countersink the screws slightly to ensure your child cannot be caught on the screws.
Cut 1/2-inch-thick furniture-grade plywood to the dimensions in obtained in Step 1 and then place the plywood on top of the bed frame. This creates the platform for the mattress to rest on.
Drill evenly spaced 1/4-inch holes in each corner and approximately every 12 inches down the sides. Position the holes 1/2-inch inward from the edges of the plywood.
Drive 1/4-inch-diameter-by-2-inch long wood screws though the holes into the plywood and into the framing.
Stain or paint the framing to your desired color and wait for the paint or stain to dry before moving on.
Place the toddler mattress onto the plywood top.