So, to start with, I painted brown lines across the board. To do them straight, I lay a ruler down half a centimeter below where I wanted to paint, and then painted a line keeping an even gap between ruler and paintbrush (so paint didn't get on the ruler), and it worked pretty well.
Then some brighter water closer to the shore
then brightest water at the water's edge
then small grit flocking for the shore
then I decided to undercoat the board in dark green, where the grass will get glued on. I realised afterwards that it would have been a lot easier to have painted the whole board green AND THEN paint the grid lines.
then started gluing on the grass. I used a wide flat brush, and painted glue onto the squares 6 at a time, then covered with piles of grass, then tipped the board on its side and tapped it against the table so the loose grass falls off.
So the idea is, I have shrubs, trees, rocks, etc, and I can just sit them on the board in a square, to create different types of battlefields.
I decided a last step would be to gloss coat the water so it's shiny and reflective :) Here's some photos trying to capture the reflectiveness of the water.