Sand the pine boards until they are perfectly smooth and even. Use the corners of the sandpaper to sand down tiny imperfections, bumps and ridges until the entire surface area of the pine board is flat, smooth and even.
Wipe down the pine board with a damp cloth in order to remove any dust that you created when sanding. Make sure the board is smooth and clean. Allow the board to dry completely before moving on to step 3.
Coat the board in sealant. Apply the sealant to the pine board using a paintbrush, going over each surface of the board evenly. Make sure that your sealant application is consistent across all parts of the pine board so that all parts of the wood will absorb the stain to the same degree. Allow the sealant to dry according to the manufacturer's instructions.
Sand any part of the wood that has become uneven because of the sealant. Wipe the board down again with a damp cloth and allow it to dry.
Apply the stain of your choosing to the now-sealed pine board. Work from left to right, moving evenly across the surface of the board. Once you are finished staining the pine board, allow the stain to dry according to the manufacturer's instructions. If you wish to apply a second coat of the stain, do so.