Use some 100 grit paper again and sand the back side of the doors smooth. You want to get rid of any Irregularities on the back side. They need to be the same thickness all over so the framework you build later will fit snug to the surface.

Using some 1/2" x 3/4" balsa build two (2) frames for your doors, thes will be the door jambs. Make the opening inside each frame 1/8" smaller than the doors. Extend the bottom of the frame about an inch beyond the verticals as shown.