Measure the outside dimension of your manger back. Mark that measurement on the proscenium back. (This meassurement will fall almost parallel with the lines formed by the portico side tabs, depending on the paper thickness you've used to glue your print-outs on.)

Making sure that you've centered the "back of manger" measurement, draw a vertical line on the back of the proscenium at each end of this measurement.

It would help using a right-angled triangle to do this. This will insure that your creche is aligned when folded.

Using one of the lines you have just drawn, attach the manger to the back of the proscenium by applying glue to one of the manger tabs. Make sure this tab lines up with your guide line.

Place manger portico-side down on a flat surface and press on the tab to insure a good bond.

With manger portico-side down (in othe words with your creche folded) check to see exactly where does the other tab come into contact with the proscenium. Does everything look nice and square? Are all the panels nice and straight with no bends or creases?

Great. Now apply glue to the last tab, and attach to the proscenium bearing in mind that it might not exactly fall right by the guide line you drew.

This will be due to slight variations in your gluing and varying paper thicknesses. But don't worry. Everything will turn out alright.