The grid for the ground is made by making sure each side matches up to one another perfectly.
Meaning the pixels have to match up. So basically take one side of a grid's selection, move it onto the other side and delete the other side of the grid so that they match up perfectly with the pixels of that other side. Do the same for the top and bottom and make sure all four side click into each other perfectly with out any cross over pixels, and if all matches well, you now have a template grid box block to work with. To make texture match up. Work with two boxes next to each other paint in the middle of the two. Then will the selection of one block side copy that half and paste it onto the other side of the far left side. do the same for top and bottom, make sure that it doesn't look odd when blocks are built repeatably. Usually something evened out will work. If you want to make your blocks bigger, just connect four and work from that as one block depending on how big you want them.