D, every two months or so, someone asks that question,
six people try to explain it with varing degrees of
failure, and nobody is enlightened. It’s a simple
thing that is very hard to describe. I suggest you
go to http://www.taunton.com and order a copy of Craig
Saveage’s video Installing Trim where he demonstrates
the technique.
BTW, I don’t mean to discourage anyone from trying
to describe it if your so inclined.