I think that is an excellent start.
You probably need a grid. In Gimp, this is how I do it (there's probably a much better and easier way – please let me know if you find it).
1) make a new transparent layer over your current layers, and call it GRID. Then fill it with white, so you can longer see you map.
2) Go to FILTERS > DISTORTS > MOSAIC.
3) In the mosaic options select SQUARES, tile size 100, tile spacing 1, tile height 1, tile neatness 1, colour variation 0. Make sure to untick COLOUR AVERAGING, or it will be a disaster.
4) Now you have the opposite of a grid, with white squares over your map. Select the 'Select with Colour Tool' in the toolbox and select the white colour, then press delete. Now invert the selection (SELECTION > INVERT) and drag the black colour (or whatever colour you like) onto the main image. Now you should have a black grid over your map.
5) Decrease the opacity of the grid layer to 30% or whatever you like with the slider, and tick the LOCK box in the layer menu; now you can drag any colour onto the layer and it will change the colour of the grid only.
6) You can make your grid bigger or smaller by scaling the grid layer.
Hope that helps.