The few times it has been an issue I have installed them. Don't quite remember how but with this whole gravity thing we have going I must have used some type of adhesive. I do remember it not being a big deal.
The carpenter usually did, but when we had to, we would. The carpeneter used polyseam seal. I guess it has give that carpenters like. It'll flex instead of crack over time. Then I just used 2 screws to hold it up while it dried. Then it was left up to the painter to dab and paint to screw (along with the medallion.
We normally put them up. Securing them depends on the size. The small ones (12-15") we do with an adhesive like EE or shortcircuit use. For the big ones (20-36") we glue and put in a few drywall nails near the outside edge. Since they're often styrofoam we use a counter sink to make the indent first then SLOWLY run the screw in to a level of (just barely) below the surface. We get the painter or drywall guy to fill the holes before they paint. Seems to work and nobody has complained about filling the holes for us.