My generator has a 6-R receptacle, but my transfer switch is an L14-30 plug. How do I configure the wire to connect the two?

You do not. A 6-XXR receptacle is straight 240v which does not have a neutral so there is no way to get your 120v part of the circuit.

An L14-XX is a 120/240v 4-wire which has two hots, neutral and ground. If your genny does not have this it cannot be done.