A plastic window frame trim is for installation of a window in a corrugated metal building wall having a cut opening with a top edge, bottom edge, and side edges, each of the edges being backed by framing members. The window frame trim includes: a frame header J-channel with a flange; a frame footer J-channel with a flange; a pair of nailing fins; and fasteners. The trim header J-channel, footer J-channel, and nailing fins are attached to the window frame. The window frame is lifted so frame header J-channel flange slides between the top edge of the metal building wall and a framing member at the top of the cut opening. The window is lowered so footer J-channel flange slides between the bottom edge of the metal building wall and a framing member at the bottom of the cut opening so that window frame rest on the window sill. Fasteners attach the window frame to the framing members through the metal building wall. Trim caps snap over the nailing fins to finish the trimmed window.