Firstly... Welcome Aboard!

Both are nice maps, the water color especially so.

How to add a grid to a battle map will probably depend on what software you are using. I can think of at least two ways off the top off my head on how to do it in Photoshop, at least one should work in most raster-based graphic editors.

Create a new empty layer than fill it with a grid-pattern. There are programs for generating these grids available on the web, but I'm pretty sure there are several examples posted here in the map elements forum.