Did they aid the Allied landings via guerilla operations on the 6th June, or did it take them at comparative unawares? I presume the risk of the Germans finding out about precise landing points trumped minor assaults that the resistance could have taken out on the day?
As little as possible.
It was not the role of the resistance to immediately become soldiers. They generally didn't have much in the way of weapons or ammunition plus they lacked training and command structure to interoperate with the landing force.
What they did have to offer was a really good excuse to be behind enemy lines, since they already lived there. They could pass information, and conduct sabotage.
There is a very nice first person account of the experiences of Andre Heintz at http://www.bbc.co.uk/history/ww2peopleswar/stories/85/a2524385.shtml . He was active in the Caen resistance from Dunkirk to D-Day.
Commands from off shore were arranged in advance in the form of phrases that would appear in the French language segment of the BBC broadcasts. In his case, he received the message to be ready on June 1, and the message to act on June 5th. Some members were sent to sabotage rails and communication. His assignment was to watch the local German headquarters for unusual activity.
Note that he had no idea when or where the landing was going to be, although even non-members had an idea that something was happening on account number of planes going over.