For horizontal siding install f channel trim at the soffit and gable rake.
How to add vinyl siding at the bottom.
Nail every 12 inches on center.
Then measure from the eave to the bottom of the existing siding.
1 2 inch 1 3 cm plywood or osb are common substrates and these are generally covered with roofing felt or another moisture barrier prior to siding the walls.
For vertical siding install j channel trim.
If the top is mitered miter the sides as well.
If this distance is divisible by 8 in the finished width of a piece of siding you re in luck.
At the bottoms of windows install under sill trim for horizontal siding and j channel trim for vertical siding.