A: As the saying goes: “Garbage in, garbage out.” Normally, a planer can’t take the twist out of a board; it merely makes the top side parallel to the bottom. To get a board flat without a jointer, fool the planer into thinking the bottom of your board is already flat.
Build a sled from two 3/4-in. medium density fiberboard (MDF) shelves. These shelves are cheap and widely available. Two of them make a really stiff sled. Shim the high spots with playing cards taped to the sled. Using shallow cuts, run the board through the planer until you’ve cleaned up the top side. Then flip the board over and plane to thickness.