It has been my experience that moving a pinball machine on your own isn't really that bad, but I'm 6'4" and 220 pounds. I've never moved one up or down stairs on my own, but I could see taking one up or down a flight.
My biggest tip if you read nothing else is not to remove the front legs until it's in the vehicle.
With that said, the steps I use are as follows.
-Use a dolly with big inflatable wheels if you have to move through grass... a refrigerator dolly will sink
-Remove balls, tilt bob, coin box, and other loose items inside machine before moving.
-Fold down top box, and strap to cabinet leaving cardboard or blankets in sensitive areas.
-Pick up rear of machine and slide a bar stool under the rear of machine
-Remove rear legs and LABEL for next owner.
-Gently set rear of machine on ground.
-Tip machine onto it's bottom.
-Slide dolly under bottom of pinball machine, with handle between legs.
-Strap pinball machine to dolly (with lots of padding)
-You can now hold the machine by the dolly or by the legs when moving.
-If you need to rest you can rest the machine on it's front legs/dolly wheels while moving.
-Wheel machine to vehicle.
-Rest machine on dolly wheels/front legs.
-Remove straps keeping machine on dolly
-Lift rear of machine into rear of vehicle (with padding).
-Push machine into vehicle until you can let go.
-Remove front legs.
-Label front legs.
-Secure pinball machine.
-Do steps in reverse at final destination.