1. Keep the ball on the deck
Being a David Moyes side, it would be all too easy for West Ham to launch crosses into the box as an initial gameplan to destabilise Chelsea. And to be fair taking advantage of height and size is a commonly practiced way of levelling the playing field between sides as disparate in talent as Chelsea and West Ham.
However it would be ill-advised on this occasion because Chelsea are very good at clearing the ball in the air. The Blues have a solid back-line built for getting rid of crosses.
In fact through the entire season they have conceded just 1 headed goal from an opponent in the Premier League (ignore the Champions League because, with the greatest of respect, West Ham are no Atlético Madrid), so if the Hammers do get the ball out wide send that cross in on the deck.
2. Set-up to man-mark Eden Hazard
Chelsea are a superb side with several game-winning talents, but none are quite as impressive as Eden Hazard. The Belgian is a dazzling creative and goalscoring force, so must be accounted for. West Ham can't simply defend well against him; they don't have the discipline.
No, what West Ham should do is set-up in a 4-4-1 basic shape, and have those players operate as a normal side. Defending deep in numbers then breaking forward to try and knick a goal on the break. That leaves one man free, and Pedro Obiang is the obvious choice for this role (he's won an impressive 20/31 tackles), to follow Eden Hazard all around the pitch.
Not saying he marks him when West Ham don't have the ball, no, even when the Hammers are in possession, Obiang sticks with Hazard. The only time he leaves him is when he wanders behind the last defensive line, as that could disrupt any West Ham offside traps. Otherwise: stick to him like glue. Manchester United showed last season how effective a system this can be to both keep Hazard in check whilst also stymieing Chelsea, so use it to your advantage.
3. Get in tight with Alvaro Morata
The final trick West Ham could use is to simply get in tight with Alvaro Morata. Don't allow him space and time on the ball. Defenders had gotten used to standing off Chelsea's striker because the likes of Didier Drogba and Diego Costa relished the physical contest, you would be in their territory.
But Morata is a different kind of forward. A more elegant kind of player, more interested in skill than super-strength. Morata has been dispossessed 33 times this season, and is not a player up for a real battle. By sticking close and making it physical, West Ham will put him on the back foot and give themselves a huge advantage.