Is it a plain, rough sawn board? If so it's usually best to start with a
scrub plane. (If the board is reasonably flat, or machine planed, you don't
both with a scrub.) Since you don't have one, my favorite of the three you
do have would be the #5. It's kind of an all purpose plane. I use mine for
flattening boards but I usually don't start out on a rough sawn board with
it. That doesn't mean you can't. Make sure you have a piece of chalk
handy, and a straight edge or combination square. Using the straight edge
at various angles (or the combination square blade) across the surface will
help you spot the high ground, which you mark with the chalk. Plane the
chalk marks away and recheck.
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