A flashing and method for installing a window or other portal in a wall opening is provided. The flashing includes a base member with first and second portions that are angled so that an outer surface of the first portion can be disposed against a sill of the opening and an outer surface of the second portion can be disposed against one of the jambs of the opening. A front face plate of the flashing extends perpendicularly from the first and second portions of the base member, and a plurality of channels in the base member are structured to direct water toward and through the front face plate.