This is just an idea off the top of my head:

Decide how large the model is going to be. and find a cardboard box with those dimensions (at least the length and width, the height isn't really important, as long as it's high enough for the trench walls.  If toy soldiers are being used, try to make it the appropriate size for the figures. 

Coat the inside of the box with vegetable oil or shortening (a thin layer) as a mold release. 

Mix up what would seem to be an appropriate amount of papier mache, and start pressing it into the box.  The thicker the papier mache, the longer it will take to dry.  If you want a fairly thick trench wall, you might want to make an interior (invisible when finished) wall of sheets of cardboard or styrofoam, and pack the papier mache over that.  You might want to wrap the sheet styrofoam in wrapping paper so the PM will stick to it, if you think you will have trouble getting the PM to stick to the styrofoam (you shouldn't have any problem with cardboard).

I would not use PVA glue (white glue) for this project, as it dries much more slowly than wallpaper paste + water or flour + water.

If you have a fan, place it in front of the PM so the air blows over it, which will help it dry.  If you used styrofoam or PVA glue in the model, you can't put it in the oven, but if you are using all paper products, you can.  Turn on the oven to the absolute lowest setting (200 F/93 C on my oven).  Dry paper will start to burn at 451 F/233 C, so if you stay away from those temperatures, you should have no trouble.  Wedge a potholder in the door to leave a crack for moisture to escape.  Check every few hours, depending on the thickness of the papier mache.

When dry or sturdy to the touch, carefully peel the outer (oiled) box from the model.  You will probably have to continue drying the project once you do this, as the PM closest to the cardboard box will be the last to dry.

Paint when dry.  A light dusting of dry sand or soil to the wet paint may make it look more realistic.  (Try on a sample before you do it to the model).

Good luck!

Sue