With this in mind, you need to decide what features on the map with effect your campaign. Before you can do that, there's a crucial decision that needs to be made and that is whether it should be a grid map or a freeform one.
Grid maps are simply maps with either a square or hex grid superimposed over them. The grid serves as a guide to movement and position. You simply track which grid an army is in and it's movement is based on how many spaces it can move per campaign turn from one adjacent space to another. Individual spaces can also be given various effects to represent wooded or urban areas and these affect any army in that space.
(From http://www.welshpiper.com - Used without permission)
Freeform maps are a bit more organic, they don't have grids to guide movement or define what areas are affected by what terrain. Instead, armies can move around the map much like models do in a game of 40K. How far they can move in a campaign turn depends on their movement rate and the effect of any terrain they might be passing through. It's quite common for various features on the map to have an area of effect, for example, any army that moves within half an inch of a wood is slowed down, so without the definition of a grid, you'll need to decide what areas are affected by what terrain. You'll also have to decide things like how close two armies have to get to each other before they battle, do they meet or just move within an inch of each other?
(From http://electronicsetc.50.forumer.com - Used without permission)
Freeform maps are far more realistic than grid maps but they do require a lot more work in setting up and tracking armies during the campaign. As always, which you decide to use is down to what you want out of the campaign and how much effort you want to put into setting up and running the campaign. Once you've decide which type you're going to use, you can then start adding the detail.