To install aluminum soffit, start by nailing the J-channel in place, installing trim pieces and slipping the soffit pieces into the trim before nailing them. Finish off by installing the facia to protect wood subfacia, and cover the outside edge of the pieces.
Drive a 1-1/4 inch shingle nails into the soffit framing to nail the J-channel in place, paying close attention to the corners. The channel holds the ends of the soffits along the house and the short sides of the soffit. If your house does not have framing in place, use a piece of F-channel along the house.
Soffits that are wider than 12 inches need additional wood support pieces down the middle to prevent the soffits from sagging. Install the first soffit panel, using a carpenters square to install it perpendicular to the house with the groove side toward the direction you are installing. Notch and fit the first and last panel around corners and trim. Then install the panels in between the ends.
Install the fascia using a piece of undersill trim to hold in place the top edge of the fascia. Alternatively, you can hide it behind a piece of drip-edge or other roof-edge flashing. Trim the top of the fascia so that it fits between the soffit and the edge of the roof.