Chop it up
Take a bit of polystyrene. Packaging is usually better than insulation batts, because the beads tend to be smaller which makes it tougher. You could use a hot-wire cutter, but when cutting small bits a sharp scalpel or a good quality serrated kitchen knife will be fine.
Stick it together
Polystyrene can be easily stuck with PVA, but it may need reinforcement - pin the joint with a couple of cut-down cocktail sticks. Alternatively you can gum it together with tile adhesive.
Rough it up a bit
Pick a few chunks out of it with your fingernails, to make some shell craters. This looks particularly effective at the edges.
If you want your building to look awful, then leave all the nasty polystyrene beads showing. Assuming you don't, then there a couple of ways you can tackle this. My favourite is tile adhesive
, which is stickier and tougher than most fillers. It also comes ready mixed, so it is quick too. Smear a thin layer all over, using your fingers and a spatula. Then sprinkle 'broken' edges of the concrete with sand
and small stones to get a rough effect.
If you don't have tile adhesive then you could use filler. It's best to mix this with a bit of PVA, which will slightly increase it's strength, however, it will tend to crack off the polystyrene if roughly handled.
Debris is really a whole topic in itself, but you could start with some of the following:
- Chunks of polystyrene (textured like the rest)
- Stones (remember to use angular ones, not round pebbles, which just won't look right)
- Plastic odds and ends
- Cardboard with one face stripped off so it looks like corrugated iron.