I just started to look a the board face and placed two aluminum angle bars at each end of the board to use as winding sticks. The winding sticks will help exaggerate the warp or twist in a board. The next thing to do is look at the winding sticks in a stooped position to make your eyes level horizontally to see the tops of the sticks. Bring your eyes up and down, using your body, not your eyes or head. Line them up at the tops and see if one side is lower or higher than the other. The longer winding sticks are, the more easily the exaggeration can be seen. In the photo below, you can see that the far right corner in the back is higher than the front stick. This means that there is a twist on that end and that is higher and that needs to be planed first.
The next thing to do is to plane all the high spots until you level out with the lowest spot and plane it flat and true.