I was in the siding business for 20 years. I would use a home made water level about 10 to 12 inches up from the bottom of the wood. If a wall was over about 30 feet, I put a mark every 30 feet or so. That is because a chalk line, even with nylon line, will start to sag beyond 30 feet. Snap a chalk line connecting the dots. With a level line, I would measure to the top of the siding area and to windows to see how level the building is. If the building and windows are a little out of level, sometimes it is better to run the siding with the building. In that case, make a new line. Also, if the windows and top panel will be in a bad place on the siding panels, there is an opportunity to start a little lower and have things work out better. Then I would measure down from the line to the starter strip and nail on the starter. Something like hook my tape on the lock on the starter and it is 11.5 inches below the line or whatever. Easy stuff if you think about it. Starter on, corners on, blast on the siding.
I would also measure up to the windows to see which panels they would fall in. I like to have window cuts in a panel, not spanning two panels. I don't want to repeat where an overlap is until three courses are separating them. I don't want the overlaps to look like steps. It would go something like start with a full panel on the first course, half panel on the second, three quarters panel on the third and quarter panel on the fourth, then repeat. If the windows require, that pattern can be adjusted such as starting with the half panel, then three quarters and so on. No pieces less than 18" and no small pieces in the middle of the wall. Laying it out makes those separations of seams work with the window cuts where they should be. I would mark the wall with a magic marker so all the thinking was done before the first panel was hung. Then I could turn on the radio and enjoy hanging the whole wall.