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What is the best way to install crown molding on plaster walls?
use multiple layers. use a ledger made of 1" stock (X 6" or larger) depending on crown size. when secured to wall it will feather out the irregularities over length and the valleys can be caulked. you should also use a backer cut to the pitch of the crown which provides for top and bottom nailing.
First find and mark the stud location, then cut a 1x as shown below that fits in the opening behind the crown. Nail the 1X into the studs then install the crown nailing it to the 1X.
I agree with Jack's suggestion to use the 1x2 and cut 45degree angles on each end.
Assuming your ceiling is relatively uniform, you might want also to snap a chalk line on the wall to get the bottom end of the 1x2's in a straight line. It makes the back of the crown mold fit flat against the backers. I've used this method and it worked very well for me. I screwed the backers to the studs with long drywall screws.