Cut an irregular piece of mounting board or thin plywood. A couple of
the rocks above were based on thin card, but this warped a bit when
the PVA was applied, so I wouldn't recommend it.
The best way to stick
the stones down is with a blob of tile adhesive
which will also fill any gaps beneath the stones.
Otherwise you will need to fill
the gap with plaster or filler.
This can also be used to make the rocks look semi-embedded in the ground.
To help make the rock look more natural, put some big patches of
PVA around the base and sprinkle them
with plenty of small stones. These can be painted in the same way as
the rest of the base.
The best thing to do is to paint them as little as possible in order
to let their natural colours show through.
If the rocks are very pale then they may benefit from a dark brown
wash, but with these I didn't bother.
The only painting that these needed was a quick drybrush with
cream coloured paint. This was done at the same time as step 4) of the
basing as described below.