Well if you are attacking from the UK you have the channel between you and France.
Just like on D-day you need to load those troops into transports and amphib assault into an occupied coastal province that has a beach. (most coastal provinces have a beach, some don't, click on a province and you should see an icon telling you if it has a beach or not.)
You need 1 transport per land divisions, whether it be infantry, armoured, calvalry etc.

Some things to be careful with amphib assaults.

-Be sure to escort transports, they are very susceptible to any type of attack. If one goes down you loose the transport and whatever was on it permanenantly.

If possible, land at a beach that isn't occupied by an enemy unit.

Use Battleships, Carriers and Heavy cruisers to do shore bombardment if you can. (this is only necessary if the province you are landing at actually has enemy troops in it)

If you attack an enemy province that has an enemy unit a battle will ensue. amphib assaults are pretty tough for the attacker. If you assault with regular infantry you will have penalties. Also if you land more than 7 divisions at once you get severe stacking penalties.
If you attack with marines there are no penalties or the penalties are minimal. If you attack with paratroopers penalties are minimal if any.

If you land marines, infantry or paratroopers make sure you have a fleet with transports available for each land unit attacking the province. If you are defeated the unit will retreat back to the transports and you'll be able to take them home to fisht another day. If however the transports went home before the battle ended all those units will be destroyed.

Any attacker will loose lots of their Organization level while attacking (that green bar on your units) If you have low or no Organization the units will be defeated almost immediatly or retreat. SO as soon as the beach falls try and have reserves offshore to immediatly land in the province when it falls to have fresh troops for defence. If there are enemy troops in the provinces bordering the one you successfully assaulted they probably will counterattack. If you only have the unorganized unit that captured the beach they surely will run away pretty quick.

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Never, never, never believe any war will be smooth and easy, or that anyone who embarks on the strange voyage can measure the tides and hurricanes he will encounter. The statesman who yields to war fever must realize that once the signal is given, he is no longer the master of policy but the slave of unforeseeable and uncontrollable events. -- Sir Winston Churchill