A wood planer shaves off the top layer of the wood to expose the good wood underneath. It is very important to check the lumber for any hidden screws or nails. The plane spins at very high-speed, and if it hits a metal object in the wood it takes a notch out of all the blades. From then on you will see a line down anything you plane where those blades are damaged. You can run the wood through again to take off the line but if you get to many dings in the blades you will have a real problem. I start with the board just clearing the blades and work the depth down from there. It usually takes at least three passes to get it looking good. Remember that you are going to have to plane all the boards to the same size in order to avoid having some boards sticking up above others. Decide from the start which side is going to be the top. I usually run the bottom of the board through the plane at least once to take off any high spots but you don’t have to worry about cutting it down to fresh wood as it’s not going to show. A plane makes big piles of chips and sawdust very quickly. Have a plan for dealing with it. I rototill all my sawdust into the garden so it’s recycled.
(See my Instructable “Shred and Till”)
Oh, and watch out for lead paint on old boards. If you suspect that they may have it dispose of them. If you plane them the lead will go everywhere in the fine dust from the planer