Measure the width of the opening under the eave of the roof with a tape measure. Most openings will measure around 12 inches. This will tell you how wide each piece of soffit must be cut. Run a tape measure along the house and add the measurement of each side. This will determine how much soffit and fascia will be needed.
Lay out all of the soffit on a flat surface. Measure and mark the piece at the width measurement with a black marker. Line up a T-square at the measurement and draw a straight line across the soffit. Repeat the process every 12 inches all the way down the soffit and for all of the other pieces of soffit.
Cut along each black line with a pair of tin snips. Position a ladder at the end of the house and slide the first piece of soffit horizontally into the J-channel on the back of the house. Hammer the soffit in place along the front of the house with roofing nails. Insert a roofing nail at every groove on the soffit and hammer it up into the rafters above.
Butt the second piece up to the first and install the rest of the soffit in the same manner all the way around the house under the eave of the roof. Line up the first piece of fascia at any corner of the roof and slide the 1/2-inch lip of the fascia under the drip edge on the top of the roof.
Hammer the fascia in place with roofing nails. Insert a roofing nail into the center of the fascia every 6 inches down the entire piece. Overlap the second piece by 2 inches over the first and continue installing the fascia around the entire house. Cut it to size with tin snips, if necessary, at the end of each side.