Cut a 1-inch square off each corner of the 28-inch wide by 52-inch long section of 1/2 plywood. Measure in from each corner 1-inch along each edge. Connect these marks to create one-inch-by-one-inch squares at each corner. Use a hand saw to cut out these four one-inch square sections.
Measure up 10-inches on each of the four 42 1/2-inches tall by 1-inch by 1-inch solid wood posts. Mark all four posts along one edge at this height.
Apply wood glue along the 1-inch edges of each square cut out in the 28-inch by 52-inch section. Insert one of the 42 1/2-inch posts into each 1-inch by 1-inch cut out. Line up the bottom of the large plywood section with the 10-inch mark on each post. Hammer one 2-inch nail into each outer edge of each post, two total nails in each post.
Measure and mark 2-inches from one post. Any post is fine to start. Measure and mark 1-inch from the 2-inch mark. Repeat this 2-inch then 1-inch process the entire way around the crib base.
Insert one 1-inch by 1/2-inch by 32-inch solid wood post at each 1-inch mark to create a 2-inch space between each post. Nail each solid wood post in place with two 2-inch nails through the bottom of the crib base driven upward into the posts.
Miter the corners of the 28-inch by 1-inch and the 52-inch by 1-inch sections. Lay each piece on a flat surface so they stand 1/2-inch above the surface. Measure in 1-inch from each corner along the side closest to you. Draw a line from the 1-inch mark back to the opposite side corner, and cut along the line with a hand saw to create a 45-degree angle.
Apply wood glue to the top of the 28-inch side slats. Line the 1-inch by 28-inch solid wood piece along the top of the slat so the mitered edges face into the crib. Drive two 1-inch nails down through each slat, and one 2-inch nail down through each corner post. Continue to line the top of the crib with the mitered solid wood sections until the slat tops are concealed.