I am installing 1 1/8 pine cove molding on an exterior porch. How do cut the corners to insure a good fit?

There is no guarantee that the old porch is square so don't go setting your miters for 45 degrees. I would not bevel any of the joints both inside and outside corners. It will be easier to nail the first board flush and then hold your second board as tight as you can and then scribe a line where you will then cut.

Use a coping cut for inside corners. Using scrap pieces make test miter cuts for outside corners. Little plastic angle finders at Home Depot work better than one would think.

Are you painting or staining? Painting & filling can hide a lot! :-)

There are two ways - but I prefer coping rather than mitreing. It gives you a bit more flexibility when the angles aren't 45.

Cut one piece square and install. Mitre the second piece and then with a coping saw, cut away the mitre. This is harder to explain than to do... but it will leave you with a piece that should fit exactly when you push it against the square piece.

here is a link on how to cope joints (http://www.taunton.com/finehomebuilding/PDF/Free/021164116.pdf)

A lot really depends on the style of the board. My directions were asssuming that you were using a plain flat 1x3 or 1x4 pine board for a simple cove. If you are planning to use a fancier crown then coping and mitering would be necessary. What were you planning?