It should be easy. Lap or clapboard siding comes with a rabbet in the bottom edge, which sits directly on the piece below, so keeping the siding level and consistent all the way up the side of the house should be a breeze. But if you plan it well, and keep an eye on it, you could find yourself off at key points – such as the bottom or top of window trim or at the soffit.
If the building is reasonably plumb and level, start by installing the bottom row or a skirtboard on the level. Then, begin installing subsequent rows of siding, sitting them tight at the joint. Once you get 18″ or so below window casing, measure from the top of the last row to the bottom of the casing. If the distance is the same, great. Simply keep going. But if it’s off at all, start adjusting your rows now.
Adjust the spacing between the butts by small amounts (up to a quarter of an inch) for each row, planning so that by the time you hit the trim, the siding is completely parallel. No one will notice the small increments.
As you work up the side of the windows, keep an eye on things, too. It stands to reason that the uppermost row of siding at the casing should be at the same height on either side, but somehow this doesn’t always happen. As you approach the top, measure the distance, and adjust the spacing so each row is the same height by the time you reach the top.
Same goes for the soffit. As long as you keep checking and adjusting, your siding will look perfect from bottom to top.