Begin with the trim
Soffit panels are cut 1/4 in. shorter than the soffit depth to allow for thermal expansion, then trapped with site-bent aluminum fascia trim and vinyl window casing. The window casing captures the top of the siding panels and mimics a frieze board, which looks better than narrow J-channel.
Vinyl siding depends on a few special trim pieces that either hold everything together or hide the edges and ends of the panels. Undersill trim, for instance, is installed under windowsills or other horizontal projections and grasps the top edge of the siding panel below it (see Installing window trim). J-channel receives the ends of siding panels where they abut windows, doors or other walls. Most pieces of vinyl trim, though, go by the same names as their wood-siding counterparts: soffits, fascia and corner boards.
I start by installing the high trim (rakes, soffits and fascias). For rakes, I usually bend aluminum coil stock on a sheet-metal brake to wrap the face and bottom edge of a 1x rake board. Manufacturers also offer vinyl-coated aluminum and all-vinyl trim stock that are bendable and give excellent results. The rake board is furred out with a board narrower than the rake to create a space for the siding ends--sort of a site-made J-channel. Prep for soffit installation begins as I frame the house. Along the tails of the rafters or trusses, I install a 2x subfascia, then hang the soffit panels and fascia on that. Where soffit panels meet the wall of the house, they are supported by J-channel, undersill trim or window casing.