First thing is holding the mill back, either with a come-along or
winch. You mentioned you have this type of equipment so the trick is
figuring out how to get it in place and lined-up with the stairs; if
you do this you can do it all yourself (I like to have my wife spot the
rollers for me when I do this). I'd put 1xs on the floor leading to
the stairs and have 1" or so steel rollers under the machine. If I was
going to leave the table on I'd have the rollers under it at an angle
so as I rolled forward the machine would be at the angle which would
allow the leading end of the table to enter the doorway first, go
forward a little, then straighten the machine to match the rollers.
You can jockey the machine around a little by running the table one way
or another. If there isn't enough room on the other side of the
doorway to straighten the mill out I'd suggest taking the table off as
you should have done this type of move before before you attempt to
keep the machine at an angle as you go down the stairs/ramp by
yourself. Sometimes you can get away with just removing the handles or
the lead screw as an alternate to taking off the entire table.

The reason for wood is twofold: First it won't tear up your floor so
you don't need anything underneath such as 1xs (hard floor that is).
Second, and most important, is when your coming down on an angle the
tendency of the roller you place, on the front of the machine or skid,
is to try to scoot back under the machine; wood rollers grip better.
We would sometimes consider them expendable.
Wish I had $1 for each time I took a Bridgeport up or down stairs; I
think I could buy us BOTH lunch (at least at McDonald's.) Sometimes
we'd get lucky and could run the wagon planks from the truck right up
to the edge of the stairs.

Dennis in nca
p.s. You're right about the 1500 mile drive but send me an airline
ticket and we can talk.