Mark the 42-by-52 inch board with an "x" at each of the points to which you will attach a leg. The "x" should go at the center of the end of the 2-by-4. Each leg must be evenly spaced to support the top of the dining room table, and should be near one of the corners of the board.
Center a 2-by-4 on top of an "x." Use 3 or 4 nails to nail the leg to the board. The nails should go from the top of the table down into the leg. Repeat the process for the other 3 legs.
Fill the nail holes with wood putty. Use sandpaper to smooth out the putty. Sand the remainder of the dining room table to eliminate rough spots.
Stain the entire dining room table, including the legs. The stain may be transparent or colored depending on what look you want.
Wait for the stain to dry (about 24 hours) and then apply varnish on top of the dry stain. Varnish will help impart a deep luster to your dining room table. It will also help to protect and seal the wood.