OK, here is a rough demo on how I put down mylar.  I'm not saying this is the only way to do it, perhaps there are better ways but its worked well for me (with two people)...

Attached is some pics of a playfield I picked up from Marks today!....(only kidding)

Cut your sheet of mylar to size and ensure it is enough.  I never pre cut holes for switches, lamps etc...This can always be done with a surgeons knife later (available from Bunnings)...

I peel back a couple of inches of the mylar.  Now when I did my head box decals, I cut the backing away (white bit).  Not really necessary for this...

I fold the backing paper back on its self in a nice C shape....I carefully apply the first section and with a clean terry cloth (or T-shirt) press down with long strokes from one side to the other....