These are dead easy if you have a kit like Forsham Cottage Arks supply (see link on right). Basically it's a floating open-ended box containing a plastic basket. In the basket you put sodden oasis with a smooth layer of sand and clay over the top. The oasis stops the clay from drying out and keeps it soft.
I've built my raft so I can test for the presence of water voles. The idea is you tether the raft somewhere you suspect there might be voles, leave it a few days, and then check the clay for footprints. When Malcolm Monie tried this out he got water vole droppings as well as prints (see above).