When the author does use corner boards, he preassembles them from 5/4-inch TruExterior trim boards, gluing the miter with Dap’s RapidFuse adhesive.
The author countersinks nails and fills the holes with epoxy filler.
The author typically preassembles window trim using a Kreg jig.
The author prefinishes his preassembled trim units.
Pieces of blue tape protect from marring the finished trim when it is nailed into place. Note the peel-and-stick flashing for the mounting blocks (made from 5/4 stock).