Wall area minus openings, which is straightforward until you reach the gables. Those are triangles, they sit at the height you least want to measure, and they are the part people leave out.
Measure each wall's width and its height from the bottom of the siding line to the underside of the eave. Multiply for the area, and repeat for every elevation.
Take the height at both ends of each wall. Ground level is rarely flat around a building, and the siding follows a level line while the ground does not, so the exposed height changes along the run.
Do this for all four elevations, including the one facing the neighbour that nobody wants to walk round. Siding is ordered as a total, and a missed elevation is a missed order.
A gable is the triangular section between the top of the wall and the roof ridge.
Its area is half the base times the height. The base is the building width, which you already have. The height is from the top of the wall up to the ridge, and that is the awkward measurement because it is well above head height.
Two ways to get it without a ladder. Measure it indirectly, using two readings and the angle between them, which is exactly what the mode exists for. Covered here. Or derive it: if you know the roof pitch and the building width, the rise is half the width multiplied by the pitch ratio. A 30 foot wide building at 6/12 pitch rises 15 times 0.5, which is 7.5 feet.
A dormer gable is the same calculation at a smaller scale, and there may be several.
A 30 foot gable at 6/12 pitch is 112 square feet. Two of them on a house is over 200 square feet, which is a meaningful share of a small building's total siding. They get forgotten because they are not at eye level.
Measure every window and door and subtract them. Unlike interior paint, where deducting openings is optional, siding is expensive enough per square foot that it is worth doing properly on anything beyond a small extension.
The caveat is that siding around an opening is cut, and the offcut is often unusable, so the saving is less than the full opening area. Deduct the openings, then let the waste allowance absorb the difference rather than deducting only part of them.
Siding is the panels plus a substantial amount of trim, and the trim is measured in linear feet rather than area.
Trim quantities come off the same sketch as the wall areas, so total them in the same pass. J-channel around openings is the one people underestimate, because a single window needs its full perimeter and a house has a lot of windows.
Wall widths at ground level are easy: brace against a corner and shoot the next one.
Heights are harder outdoors in daylight, because you are shooting upward at a soffit and the dot becomes hard to place in sun. Gable peaks are worse still. This is where a unit with an angle sensor earns its keep, since deriving the height from an angle and a distance avoids needing to see the dot at the peak at all.
Reflective or light coloured siding on the existing building compounds it. The outdoor picks are here, and measuring pitch, which you need for the gables, is here.
If you would rather just be told what to buy, our best laser measures are on the front page.