Tear-off and what's underneath
A siding job usually starts by stripping the old cladding down to the wall sheathing so the crew can see what's actually there: rot, failed old flashing, or sheathing that needs replacing before anything new goes up.
On an older home, this is also the point where a crew can run into the EPA's rule on lead-based paint, covered in a separate guide on this site, since cutting into old painted siding counts as disturbing it.
The water-resistive barrier
Building codes require a water-resistive barrier under the exterior wall covering on a wood-framed wall, a layer whose only job is to stop water that gets past the siding from reaching the sheathing.[1]
On a full re-side, that barrier is almost always replaced along with the siding, since the old one is typically stapled or nailed down and gets torn during removal.
Seams in the new barrier are lapped and taped so water sheds downward and outward, the same shingle-lap logic that keeps a roof dry.
Flashing at every break in the wall
Flashing goes in wherever the wall plane is interrupted: around windows and doors, at the base of the wall, and anywhere trim meets a horizontal surface, so water is directed back out instead of behind the new siding.[1]
Skipping flashing to save an hour of labor is a common cause of hidden water damage behind new siding, since the failure often doesn't show on the surface until the sheathing underneath has already rotted.
Panels, planks or shingles go up last
Once the barrier and flashing are in, the siding material goes up starting at the bottom and working up, with each course overlapping the one below it.
Corner posts, J-channel around openings, and trim boards are typically set first or alongside the field material, so the siding has something to lock into at every edge.
Trim and the final walk-through
Trim work, including fascia, soffit, and window and door casing, usually wraps up the job, since it covers the raw edges of the siding and gives water somewhere to drip clear of the wall.
A reasonable final walk-through checks that every seam is sealed where the manufacturer calls for it, that no fasteners are overdriven into the face of the material, and that the site is cleared of packaging and offcuts.
What a careful crew checks before closing the wall back up
Before the new water-resistive barrier goes on, a careful crew checks the sheathing for soft spots, delamination, or water staining, since covering damaged sheathing with new siding just hides the problem for a few more years.
That's also the moment to confirm the framing behind the wall is straight enough for the new material to lie flat, since panel and lap siding will visibly telegraph any wave or dip in the wall underneath it.
Any window or door flashing found taped over or missing entirely during tear-off is a sign the earlier installation cut a corner, and it's worth pointing out so the same mistake doesn't repeat.
Manufacturer instructions carry code weight too
Most jurisdictions let a siding product's own manufacturer installation instructions serve as an approved alternative to a generic code method, provided the crew actually follows them, which is one reason a legitimate contractor keeps a copy of that specific guide on site instead of installing from memory.
That's also why two houses with the same siding material can look installed slightly differently: fastening pattern, clearance at trim, and starter-strip details are spelled out by the manufacturer, not improvised by the crew.