What I do is first start up the APU (from Cold and Dark), then I begin boarding. Next I call baggage and fuel services. Then maybe begin catering if it was needed.
But this will depend on how it is IRL. On cold and dark starts, airlines will probably add fuel and catering before boarding, and add baggage a little before and during boarding.
On turnarounds, they have the jetway connected all of the time while on the ground (mostly). Then they will do all of the ground jobs (baggage, fuel, etc.) during deplaning and boarding. And maybe baggage if it is needed
That is what I think airlines do IRL.