Not that many home woodshops have a jointer larger than 8″ wide, and many have no jointer at all. Not to worry – a hand plane can flatten the widest board, and safely plane a part too small to safely mill with power tools.
The process begins by planing diagonal to the grain working along the length of the board in both directions. This is called “scrub planing”. Set the blade to take a deeper cut than normal, and as you see the high spots disappear, work the blade back to a finer cut (much like sanding to finer grits).
“Winding Sticks” can be used to insure that the entire surface is flat. They are two thin straight edge sticks of the same width. One is used to check for flatness along the length and width of your part, and together they check for twist.
Note that one stick has two black marks on the ends. Set one stick on each end of your part and sight across the top from front to back. The black marks should be evenly exposed, if one is more visible, then that corner needs to be planed down.
Once the part is properly flattened, you can send it through a power planer to make the faces parallel. Or, if you want the challenge, the entire part can be prepped by hand.
A marking gauge lays out a line on all four edges at the final thickness, and the second face can be pared down and smoothed just like the first. Work the face down until you reach the thickness mark all the way round, and check for dips using the straight edge of the winding sticks. Lastly, one edge can be planed smooth and square to the first face.
This process might seem slow and laborious, but really goes quite quickly once you get the hang of it. You may not often need or want to prep stock this way, but it is a handy skill to have for over- or under-sized parts.