A gable is the angled peak of a traditional residential roof. The soffit is the horizontal under-face of the roof, as it extends from the front of the wall out to the fascia. The soffit is typically made from thin sheets of luan plywood fastened to the underside of the rafters. It fits between the back edge of the fascia board and the front face of the wall. Painting your soffit before installation will limit your time on the ladder and make things go a lot faster.
Measure from the face of the wall out to the back of the fascia board. Use a table saw to cut enough 1/4-inch-thick luan plywood to fill the area under the eaves of the gable, from the bottom edge of the roof all the way to the peak, on both sides.
Fill a plastic paint pan with latex house paint about 1-inch deep. Fit a roller cover on a roller handle and fit the handle onto an extension pole. Roll two coats of paint onto your soffit panels, using a W pattern in overlapping strokes for adequate coverage. Allow two hours between coats for drying and three hours before installation.
Fit the soffit's first panel to the bottom of the rafters, with its narrow end at the underside of the roof peak. Staple up through the panel into the rafters with a pneumatic staple gun, using three staples in each rafter.
Fit the next panel, with its narrow end butted up to the end of the first panel. Continue adding full panels down the roof as far as possible. Measure and cut a piece to fill the area from the last full panel to the bottom edge of the roof. Do the same thing for the opposite face of the roof.
Caulk along the edges of all soffit panels with latex painter's caulk. Allow the caulk to dry for one hour. Use a paintbrush to cover the caulking with latex paint and make it virtually invisible.