STEP 2: FLATTEN A REFERENCE FACE WITH HANDPLANES
Place the board between the bench dogs with the arced side facing up, to avoid rocking. You may need to use shims if your board is in really bad shape. Use a scrub plane or a jack plane with a cambered iron (8 degree camber/arc). This plane is going to be doing rough work, so don’t worry about tuning it extensively.
If you have an extreme arc in the board, plane down the length of the board, removing the high center:
Before planning across the grain, bevel the edge that is farthest away from you, to prevent major tear out:
Then plane across the grain, from one end to the other.
Adjust your plane so that your shavings are as big as possible, while still being able to move the plane.
You can also take some diagonal passes both ways, to aid with flattening:
Tilt your jack plane on its edge and drag it along the board to get a rough idea of your progress toward flatness: