Hi Dave. If you want to make it really easy to set up a flight plan in P2Atc then:
In P2Atc:
- choose a destination airport
- choose an arrival airport
- press the AutoPlan button
- export as .pln (I think) to a flight plan folder
- press ‘connect’ to sim
In MSFS 2020 at the flight screen:
- load up flight plan just saved above
- choose to start on runway or on ramp
- decide time/weather options etc
- press ‘fly’
- start engine/avionics
In P2Atc:
- validate/file plan with atc
- choose airport ground frequency and copy IFR clearence if you want IFR flight
This is from memory (not at my pc) so I might perhaps have missed a couple of things. If so, then apologies 
Anyway, hope it helps a bit and does’nt confuse.
If you want to set up your own SIDS/STARS/Waypoints/Altitudes etc then you can do that too