There are several threads here if you search around about how to square
up wood. And there are as many methods as there are opinions.

The planer has no way of indexing the flatness of the face to be
exactly perpendicular (90 degrees) from the edge of the board. The most
common method (that I know of) is to do one face on the jointer first.
Then do one edge, using the newly flattened face against the jointer
fence so the edge is now perfectly perpendicular. Then use the planer
to do the other face. then rip on the saw as needed using the straight
edge.

Of course you need a jointer that is as wide as the board to do this.
Alternatively, you can just plane and rip but it is not as accurate.

Watch closely for the firestorm to follow.