Hi, even though the method s2art explained is the best way to do this, I just want to add that you can also draw 3d lines with the morph tool in the face of the door leaf, but you have to do it on both sides in order to see them in both sides of the door.
This is just a little trick that is good to know (see the white lines in the door, those are with the morph lines that are only visible on this side of the door, because I didn't draw them on the other side).
But again, I think that the best way to do a custom door leaf is to build it the way it would be build in reality, that way you can have different surfaces in the different pieces and with this have the wood grain running the correct way.