The trick to a long miter joint is flat, straight stock. And a lot of clamps.

So a guy calls and asks me to trim out a stairway that he and his buddy built.

A handful of tips that will speed your process, reduce mistakes, and yield a better-looking finished product.

When bridging an opening in the base cabinets, don't rely on the wall to dictate cabinet placement. Use a straightedge to align the faces of the cabinets on either side. 

Bring scale and proportion to a room without using armloads of trim stock.