Draw an "X" in each corner of the wooden board using a pencil. Each "X" will be the position of each leg, so ensure they are in corresponding positions.
Place a leg so that it is underneath an "X." Hammer a nail through the top of the board to secure the leg in place. To make the desk more sturdy, do this with four nails and repeat the process for each leg.
Test that the desk is sturdy by standing it on its legs. If it rocks or seems uneven, sand the longest leg until it is even.
Fill in the nail holes with wood putty and sand down to make even.
Remove any rough spots on the desk using sandpaper.
Stain the desk any color you wish. This not only improves the look, it also helps to protect the wood.