Cut out a section of hardboard that will be sufficient for the width and length of your river piece. Alternately, you could use some thick and sturdy cardboard. Now glue a strip of 1/2 inch thick foamcore or insulation foam that will act as the banks of the river to the hardboard. Leave an area between the two that will serve as the river. Some of the foam should overhang the base, but don't worry... this is a good thing.

Let the glue dry and flip the river over. Carefully use a sharp hobby knife to cut the foamcore to the exact curvature and shape of the edge of your river. Do this for both sides, and flip the river back over.

Using a new hobby blade, carve into the foam banks to shape out sloping banks. Cut slowly and not too deep with each stroke. Just sort of whittle the banks down slowly as this will lead to less mistakes. Once you have a good basic shape, use a fine grade sandpaper to smooth out any rough spots and gaps between the river bank and the hardboard below. If there are any hideous gaps or cuts in the foam you can always use some filler putty to fix the problem for good!

Leave your river for a moment and head outdoors to grab some interesting stones. Try and find ones with a flat edge as this will make it much easier to superglue them to the hardboard base. Dig out a bank and glue a rock in place or just put a few rocks along the bank. Whatever you think looks best! When you glue the rocks down, don't waste any excess glue. Sprinkle a bit of fine sand around the base of the rock and the superglue will do double duty by gluing both the rock and sand!

Water down a bit of white or wood glue and use an old brush to spread the glue mix all over the banks. In the centre of the river there is a clump of rocks, to simulate the rough water found at this spot, spread some glue around this area too. Finally dump some sand on the entire river piece. Use a mix of fine and coarse sand or whatever you think will look best.

Once all of the glue is dry, primer the entire model with Chaos Black paint. You are now ready to paint the river section. Colour the banks any way you wish and paint the river with deep blues. Streak and swirl lighter blues on top of the darker blue and let this dry. Finally, finish off the paint job with a thick coat of Gloss Varnish on the water sections only. Now go and make a bunch more and you'll have a full fledged river!