Ensure all your boards have been trued and are square, to see how to do this, see our Truing Stock guide.
For this you will need:
Lay the boards flat on a large surface such as the workbench top to decide their configuration. Then, put your boards together and align the edges on two pencils. Pinch both your boards together and place them carefully in the vise. Use a plane to take off the high spots and get them so that they are the same level.
Once you are happy your boards are straight and true with no gaps, zig zag the glue along the edge of one of the boards then use the second board to even out the glue over both surfaces.
Use two clamps to hold both glued surfaces together. Put a third clamp on top of the boards to prevent uneven pressure that will often cause a bowing up. This then helps to squeeze out any excess glue. Wipe away any excess glue with a tissue. Keep in the clamps for approximately 8 hours so that the glue has time to dry.
Once dry, cut to the size you need. Plane the newly sawn edges. When planing the end grain, be sure to avoid the ends splitting by planing in halfway then coming from the other side.
Place the clamp in the vise and use it to hold the board while you scrape the surface. Use a #80 scraper to even up the surface and remove the glue. Finish off with sandpaper.