The gen prep has the same connections as your main power cord. There are two hot connections, L1 and L2, Neutral, and Ground. The L1 and L2 connect to separate busses in the circuit breaker panel.
If you get a receptacle for a 50 amp plug and wire it to the gen prep, then you can use a 50->30->20 pigtail/adapter to connect to the generator. The generator power will then flow to both L1 and L2.
You could also take the simpler approach of using the pigtail/adapter to plug your main power cord into the generator.
You'll also want to take a look at
this article on generator neutral bonding.