Vinyl soffit and fascia has become one of the most often used products for exterior trim. It has practically replaced aluminum in new construction and home remodeling. These items are easy to install and easy to replace if needed, and will protect exposed wood from water damage for years.
Remove any existing soffit and f- or j-channel and fascia. If you have an older house, chances are these materials are aluminum. You will also need to undo the downspout from the gutter. Keep the downspout attached to the wall, but you will need to drill through the pop rivet that connects the downspout to the gutter. It typically requires a 1/8" drill bit.
Set your j-channel along the wall and nail the long side of the channel up to the underside of the soffit plywood. Use one nail every 18" or so. If there is no plywood, nail at the bottom side of each truss. Make sure to run the j-channel the full length of the wall without any gaps.
Cut your soffit pieces. Since soffit runs perpendicular to the wall you will be cutting many short pieces, depending on the width of your overhang. Most overhangs are 6" or 1'. Slide one end into the j-channel and nail the far side of the soffit to the wood fascia board using one nail to keep the soffit in place until you install the vinyl fascia. Make sure to nail in the groove to keep the soffit from buckling.
Cut your vinyl fascia pieces a little bit longer than you measure so you can trim back if needed. It's a lot better to have to trim back than to be short and need another piece.
Tuck the top of the fascia under the drip edge between the shingles and roof plywood. Nail only at the top of the fascia (about 1" from the top). This will keep the nail heads hidden once the gutters are re-installed.
Reinstall the gutters. All you need to do is nail them back in place. But the nail holes are covered with new fascia so you'll need to make sure you install them on an angle. Flat gutters won't efficiently do their job. If you have one downspout on this run of fascia, then angle the gutter to empty from that downspout. If you have two downspouts, one on either end, you'll need to install the middle of the gutter highest, then angle it down on both ends. Use pop rivets to reinstall the downspouts.