I'm trying to make a CarWars Battlemap. I need a quarter inch grid over the whole map with heavier girdlines every full inch. How do I do that?
Use Draw -> Hex or Square Overlay
Note that CC3+ doesn't allow automatic creation of grids with variable thickness. You could either make two grids, or, since it sounds like you won't have that many whole-inch squares, you could probably just change those lines manually. The grid is a grouped entity, but you can unlock groups by hitting the Locked button in the bottom right, and now you can just use Change Properties on the grid lines you need to make thicker.
Note that you normally make maps in CC3+ in real-world scale, not paper scale, so grids aren't normally specified in inches, but since this is a rather abstract map, then perhaps you should just treat one map unit as one inch in this case.
You could also create the first grid, create a new Sheet, and move it to that the sheet. Then you can assign different Sheet effects to give each grid a different look or even just use a different color. Just make sure you assign it to the Grid layer so you can use the layer to freeze or hide both grids.