You will begin gluing the three figure panels, starting with the back one (the Virgin and Child) and moving towards the front.

But before you glue the figure tabs to the right wall (as shown in the top image, left) you must insure that each figure panel will lay down flat when the creche is folded.

For this to happen, each tab must be scored and folded exactly as indicated.
Not doing so will mean these panels will be of unequal length, and this will cause one or more of the panels to slightly bend, or to not allow the creche to fold down completely.

To avoid such a possibility, glue each tab with the creche upright as shown in the top image (left), but, before the glue sets, place it down on a flat surface, with the right wall folded on top, as shown in the image below left.

You might have to apply just a bit of force, depending on your scoring, to make the creche fold flat. Make sure the figure panel whose tab you just glued will lie down without creasing or preventing the creche from folding. Adjust the tab fold if necessary, then press the tab down to insure a secure bond.

Repeat this process with the tabs of figure panels 2, and 3, each time making sure that the creche will fold flat: you'll be amazed, if you follow this procedure, why easy it is to do!