Cristo-
as Recon mentioned, there's quite a few posts on the subject.
Here's one from Faust:
https://armorama.kitmaker.net/forums/33194&page=1 and another one on some of my initial bunker issues:
https://armorama.kitmaker.net/forums/26371&page=1 Reader's Digest version:
1) cut to size either styrofoam or foamboard
2) glue in place with either a hot glue gun or Aileene's foam-tastic glue
3) then cover with your choice of 'concrete' (wall spackle, plaster, etc.)
4) and then paint (or you can add acrylic paint to your plaster mix)
5) good to go
It sounds easier to make than it is, but it depends on how detailed you want to make it (such as adding the planking effect) and how big and detailed the construction of the bunker is ( 1 room versus 5 or 10 rooms).
Hopefully this helps you out.
If you need scale dimensions or drawings, let me know and I can help you out.