Word of advice: terrain made from cardboard and styrofoam (expanded polystyrene) is incredibly lightweight, which is good for storage and transport, but bad for gaming, when a stray sleeve, careening Rhino or carelessly opened door can lead to a radical redistribution of terrain. In a game that relies on True Line Of Sight (as many do, these days), a shift of even a few millimetres can be the difference between no line of sight and dead.
If you follow this - otherwise exemplary - tutorial to make your own terrain, I recommend a couple of changes. The first is to use at least one actual rock in your assembly rather than all styrofoam. It can be hard to get a rock to stick, so there is a bit more work involved (you make up some plaster filler using PVA (white) glue and slap it around the base of the rock to fill the gaps and bond it to the cardboard; alternatively, use plasticard for the base and just superglue it on), but it will give you a bit more heft to the terrain piece. Second - and a bit easier than the first - glue felt to the underside of the card base. Assuming you play on a surface with even a little texture this will provide useful "grip" on the tabletop to prevent shifting.
I tend to use a hot glue gun to stick pieces to thin ply. A bit sturdier, and with the high density stuff with a few coats of paint, you can stack it with care.
I've had some weird looking terrain with the hot wire, loads of fun.
This tutorial will be very useful.
Cheap , quick and effective.
If you follow this - otherwise exemplary - tutorial to make your own terrain, I recommend a couple of changes. The first is to use at least one actual rock in your assembly rather than all styrofoam. It can be hard to get a rock to stick, so there is a bit more work involved (you make up some plaster filler using PVA (white) glue and slap it around the base of the rock to fill the gaps and bond it to the cardboard; alternatively, use plasticard for the base and just superglue it on), but it will give you a bit more heft to the terrain piece. Second - and a bit easier than the first - glue felt to the underside of the card base. Assuming you play on a surface with even a little texture this will provide useful "grip" on the tabletop to prevent shifting.
I've gone a bought a hot wire gun and some high density styrene insulation blocks. Never had so much fun chopping stuff up!!
I've had some weird looking terrain with the hot wire, loads of fun.