At the main menu click Freeplay and then do Mira HQ.
As soon as the game loads go to the Laptop, open the Laptop and click on Imposter.
The crewmates are in storage, balcony, admin, greenhouse, laboratory, and locker room
From here I thought the best way was to start in storage and kill Blue. After killing Blue walk South to the balcony and kill Pink. After killing Pink use the vent to the right in the balcony and go up twice to admin and kill Black. From admin walk into the greenhouse and kill Yellow. After killing Yellow use the vent behind the tank in greenhouse and go Southwest twice to laboratory. Kill Orange in laboratory.
As soon as you kill Orange a message pops up telling you that you would have won the game. Close the menu and it will respawn everyone. From here kill everyone in the reverse order until you end back in storage. I felt this minimized all the walking you had to do.
You do need to load into an online game once and then back out before going to Freeplay.
You can also change between Imposter and Crewmate freely if you decide that doing tasks becomes to boring.
You can also back out or quit at any time without worry either.
You can use any of the other maps or Imposter kills but I think Mira HQ minimizes all the walking you need to do.