Get a map where positions can be noted with a dot. Anyone with a computer has or can get an image editing program. Something as simple as MS Paint will allow people to put a dot down anywhere on the map.

The level of sophistication of the map can vary depending on where you get it or how you yourself make it. It can feature terrain elevation, a grid system that breaks the map down in a regular pattern of every X meter or kilometer, and the map can of course contain geographic features.

It's easily the simplest way to convey the most detail possible. Even in non-NRP RPs I'd consider it honestly a good investment if not a flat requirement.