At the public's disposal are collections of both modern and historic maps.
For modern maps, I rely chiefly on Google Maps and ACME Mapper. For historic maps, I cannot speak too highly of the David Rumsey map collection. The Library of Congress' digital map collection is also recommended.
Of course, there are also available more specialized collections. While researching the American invasion of Canada, for example, I've relied heavily on the online collections maintained by the Bibliothèque et Archives nationales du Québec and the McCord Museum of Canadian History.
To make a map, I rely on both modern and historic maps. In the case of the map I made of Fort Saint-Jean and vicinity, I was able to resize and "paste" a historic map (which shows lost terrain features) onto a modern map of the area.
Making maps (click to enlarge). At left, an early of Saint-Jean-sur-Richelieu has been superimposed on a modern map of the area. The early map shows a hill and stream that were important during the siege of Fort Saint-Jean, but that are not visible on modern maps of the area.
The roads and place names shown in the final map were derived from 18th Century maps of the area, such as the one partially shown below.
An early map of Fort Saint-Jean and vicinity. North is at upper left. The fort is at the southern terminus of the road (labeled C). The Richelieu is labeled Rivière Chambly. The roads that diverge north of Rivière Saint-Jean are shown going to La Prairie and Chambly. These roads remained in use at the time of the Revolution.
Once I've been able to compare old and new maps to work out where some specific event occurred, like the skirmish at Rivière Saint-Jean, I can use Google Maps' street view function to "visit" the site of these historic events. The image shows the site of this skirmish, which of course is much changed from 1775. The road heading into the background follows the route by which the British supplied Fort Saint-Jean in 1775. On September 18, 1775, the Americans under Major John Brown defended a breastwork that was built across this road (likely near the houses in the background).
Rivière Saint-Jean has been obliterated by urban development. The road that angles to the left sits on or near the site of this stream. The Richelieu (into which it flowed) can be glimpsed at right. The British deployed for battle near the spot from which this image was made.
The methods of research described above are of course better suited to some battles than others. I'm planning to write in the not-too-distant future about the battle of Longue-Pointe, which was fought near the city of Montreal. The Google street view image below was taken somewhere near the place where the British deployed for that battle and looks in the direction of Montreal (from whence the British marched). However, the landscape has been so utterly transformed by development that it's quite impossible to visual the scene of the desperate fight that once took place there.