From what I remember, it consisted of the following.

1 Dig a rough "rock shaped" hole
2 Line it with polythene
3 Mix concrete and peat substitute
4 Dump in hole and leave to dry
5 Pull out (a friend may be helpful for this step).

A similar "fake Dublin sink" can be made using a large lined box with a
smaller lined box to for the "inside" of the mould.

The peat substitute was in there to give a result that was less "concrete
grey" and which also "took" lichen/moss/clinging plants readily.

I imagine any good DIY book will give some reasonable concrete mix recipes.
You may want to choose a slightly stronger mix to allow for the affect of
the peat substitute.

Also, if you're intending making a lot, a concrete mixer is a must (hire
one). Even apparently small amounts of concrete can be back-breaking to mix
by hand (say's he from experience!).

Paul DS.