Mike Smith gave a presentation on how to flatten a board.
First when selecting wood look at the end grain
The left shows how a “flat sawn” log is cut. The end grain on the board would be in a circular pattern. As the wood dries the grain would flatten out which can lead to cracking and warping.
The right photo shows a “quarter sawn log.” The log is cut so the end grain is straighter which makes the board more stable and less prawn to warping and cracking.
Mike discussed several ways to flatten a warped or cupped board.
First, acclimate the wood to the local environment/temperature. Leave it in your shop for a few days so to help eliminate movement and cracking.
One way to flatten a “cupped board” is to run it through a joiner (with the grain) with the cupped face down until that face is flat. Next run it through the planner
To create a straight side you could again use a joiner. You could also attach your board to a known straight edge as shown here then run it over your table saw.
If you have, a board that is warped/twisted places it on a sled and shim it so it doesn’t move. Starting with the high end, feed it through your planner taking SMALL bites (do both sides).
Another way is to make a box and set the wood inside. Next, mount your router on a stiff board so it rests on the side of the box. Use a flat bottom router bit protruding through the board and move the router along the sides and taking small bites off the warped wood.
Note: The bigger the router bit the slower the speed needs to be (see manufacturer manual).