Just install the PTP file using the OPS Center.
This does include a custom SWA cabin done by Jviation. I just painted a concept livery. Check out his awesome work on other Southwest liveries in the link below.
Southwest Airlines "Nevada One" w/Cabin PMDG B737-7H4 for Microsoft Flight Simulator | MSFS
https://flightsim.to/file/35756/southwest-airlines-nevada-one-w-cabin-pmdg-b737-7h4