How did Allied forces prepare for the invasion of France?

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.