What did the allies do to make sure the D-Day invasion would be a success?

The allies did a number of things to ensure the success of the D-Day invasion.

The key thing they did is work to convince the Germans that the invasion would come at Calais as opposed to the beaches at Normandy.

The allies built an artificial harbor at Normandy through which all of the needed supplies of a successful invasion could be brought ashore. They trained and prepared for months ahead of the June landing. They assembled the largest flotilla of ships the world had ever seen. Warships pounded the coast with heavy guns to soften the awaiting enemy. They landed paratroopers behind enemy lines. They executed a sneak attack at Normandy by creating the impression of invasion at Calais.