The most obvious way that the Allies prepared to invade France was by bringing huge amounts of war supplies from the United States to Britain. The materiel was stockpiled there in preparation for the invasion.
The Allies also started to do more to help the Resistance in France. The idea was that the Resistance would help the invasion by doing various acts of sabotage that would make it harder for the Germans to respond to the landings.
Finally, the Allies put on a huge deception to convince Hitler that the invasion was going to come near Calais instead of in Normandy.
In 1942 Allies forces began to plan invasion of France. Initially they raided the French port of Dieppe on the channel where faced strong German defences and suffered heavy losses. IN light of this experience they decided that landing on open beaches was preferable than landing in a port.
Throughout 1943 preparations were made by the allies for such an invasion. They Allies assembled huge amounts of equipment and great numbers of troops in southern England.
The Germans also expected an Allied invasion along the north coast of France in 1944, but were unsure of the place of invasion. They made a chain of fortifications, called the Atlantic Wall, running along the coast. Germans brought in artillery, mined the water and the beaches, and strung up barbed wire.
The Allies decided to land on Normandy beach and establish control over it. The Germans had not very much expect the Allied landing here. The landing, called D-Day was initially planned for June 5, 1944, but was postponed by a day due to bad weather. The invasion, carried out at night used about 2,700 carrying 176,000. Minesweepers had preceded the ships to clear the water. Also paratroopers were used to capture bridges and railway lines behind German lines. By day break the troops stormed ashore along a 100 kilometer front.
Germans fought back fiercely, but they were taken by surprise and were unable to stop the allies.. As a result, by the end of the day all Allied landing beaches were secure. Allies then create an artificial harbour for unloading more troops and supplies. By the end of June 1944, about a million Allied troops had reached through these facilities.