1. Measure and draw where the river will be made. I use a system where the right side of the river is 50mm from the right edge and the left side of the river is 200mm from the left edge. That way the river tile can be placed with the river on the right side of the field or left side of the field by turning it around. The key here is to have a consistent system in place so the finish of the river on 1 tile measures up with the start of the river on the next tile. Later you can add fancy tiles where the river moves from 1 side of the tile to the other in a diagonnal fashion, or even exists of a different edge in a "L" shape.
2. Cut both lines representing the sides of the river and soften the edges to model realistic banks. Remove the waste from the middle.
3. Glue the finished sides to the base tile.
4. Paint the river bed section with water colours and add water affects if you wish. I just paint with a middle blue knowing the different in the cork used in the tile will enhance the blue paint I apply.
5. Flock and add detail as you wish. (see previous articles for the flocking I use).