This is just a simple project for a cute little mouse door. I assembled this with super glue. For the mailbox, attach the medium piece to the larger piece then just run a thin bead across the top edge of the two larger pieces and place the smaller piece across on the angle.

Attach the arching door frame to the larger of the oval topped pieces. Then attach the door spacer. This piece is what holds the door ajar to help create the illusion that the mouse is peeking around it. Apply a bead of glue at the join of the door frame and the oval topped piece and along the left edge of the spacer. Then attach the door. Finally, attach the door knob to the door. 

Attach the mouse head and hand to the door. Use the temporary adhesive of your choice to attach the door/mouse assembly and the mailbox to your wall. I usually paint the final assembly. Make sure to paint the larger oval topped piece black to help sell the illusion of depth behind the mouse.