I think you need to have Hitler realize from the get-go (i.e. 1933) that Britain won't negotiate and a cross-channel invasion will be necessary, and instruct the Kriegsmarine to devote it's research budget to developing the specialized invasion craft which would have been necessary, as well as lots of destroyers and some cruisers to escort them (no big battleships like Bismarck or Tirpitz).
The Royal Navy must be dealt with. A capability to rapidly set up mine belts in the Channel to strongly deter attack on the invasion craft by the R.N. would be needed. There is no way the Kriegsmarine can build enough large capital ships to deter the R.N., so the Luftwaffe must carry out that task. Goering is instructed to devote a significant portion of his training budget to anti-shipping operations. A viable torpedo bomber should be developed, possibly using copies of the Japanese Long Lance torpedo (once the treaty with Japan is signed).
The Royal Air Force must also be dealt with. A long-range, single-engine fighter must be developed (drop tanks for the Bf109, perhaps?) to escort the bombers, which will concentrate on radar installations, airfields, and known aircraft production facilities.
If Hitler starts all this stuff in 1933, and manages to keep the British in the dark about the preparations until 1940 (difficult but not impossible), Sealion could work.