to use a lot of force to keep them all lined up like you see in the picture below. you can use something like a hammer or just put all of your body weight on them until you get them all lined up correctly.
then wait until the next day before removing the clamps. it should then look like the picture below.
sand sand sand. start with a 120 grit sandpaper until it feels smooth to the touch. then use 220 grit sandpaper (or higher) to make extra smooth.
then use a wet rag to wipe the wood down. this brings the grain out of the wood so that you can more finely 