Install lengths of J-channel along the inner edge of the fascia. The J-channel will conceal the cut edges of the soffit lengths and will provide a watertight seal.  Your nails should be centered in the channel slots and the nail heads should remain 1/32-to-1/16 inches (0.7938-to-1.6 mm) out. Boxed-type soffits will need a second J Channel strip, running from the fascia to the edge of the house. If the soffit on your house wraps around a corner, you will need to make provisions for the change of direction.  You can do this by installing two J-channels diagonally where the corners of the roof and house meet. You will have to cut a number of soffit and vent pieces at an angle to accommodate the diagonal pieces of J-channel. Vinyl siding usually comes in 12-foot (3.66-meter) lengths. Therefore, you will need to cut these long pieces of siding to fit the measurements of your soffit.  Keep in mind that the soffit pieces should measure 1/4 inch (6.35 mm) shorter than the actual length of the soffit. This 1⁄4 inch (0.6 cm) gap allows for the expansion of vinyl siding in warm weather. Once the J-channel is installed and the soffit pieces are cut, you will be able install them.  You can do this by pressing the soffit pieces into the channel, bending them to fit if necessary (vinyl siding is quite flexible). If you're having difficulty just pressing them in, you may need to pull the channel lip back with a pry bar or locking tool to get the siding panels to fit . Once the soffit pieces are installed, remove the gutter/downspout and slide the lengths of fascia siding under the gutter apron.  Secure the top edge of the fascia pieces with galvanized or painted nails placed every couple of feet. Reattach the gutters.

TL;DR in Indonesian:
Paku channel J di bawah fascia. Pahami cara mengerjakan soffit yang berbelok. Ukur dan potong pelapis dinding vinil bagian soffit. Tekan setiap panel ke dalam channel J. Pasang pelapis dinding vinyl bagian fascia.