Gather these parts first

Flip one of the door opening plates up side down and slide the end walls onto it as shown above

Slide the other door opening plate on the other side of the end walls with both sides showing the detail facing out

Glue the base plate onto the bottom of the assembly so far

Flip it over and glue the rim onto the top

Now gather the parts above to make the removable door

Glue the two door surface plates back to back with the detail showing.  Make sure that they are lined up as closely as possible

Apply glue and slide the tall “U” shaped bars down into the slot on your door.  Ensure that the door and the bars flush on top and bottom.

Now glue the top plate onto the door

Now gather all of the parts above on your parts sheet

Glue the textured ribs into the 4 slots on both sides of the door.

Glue the central gun piece into the middle of the ribs from the previous step

Now your door can drop into the frame