When I went loopy and redid our two front rooms, I foolishly did all the chair rail part by myself. The real problem was trying to miter the joints with a cheap plastic miter box and a hand saw — I thought going slowly would allow me more precision, but it didn’t work like that. Precisely joining 45-degree angles ain’t easy; I made gaps so big I literally had to spackle them. Most aren’t too noticeable, and the overall effect of the new paint colors and the molding makes the rooms look nice, but a few of those joints are right out in the open, and I cringe.
Corner joint shows the gap before I spackled and painted it. Small planks temporarily taped to the wall -- my "extra hands" -- hold the trim piece level while I nail it in place.
So my first bit of advice is to pay someone to do this for you. If you’re still reading, my second piece of advice is to rent a power miter saw for this project, which I assume would give you more precise joints. No pre-cutting at the store is going to work, because you have to fit these lengths as you go along — walls aren’t perfect, and all sorts of variables will throw you off. (Usual advice is to start in a corner and work outward from there.)
My third piece of advice is to not mess with laser levels or chalk lines or however you were going to mark the straight line on the wall. Just take a yardstick and mark the same height all around the wall. Fast; and no futzing with tape measure error. And if you somehow marked a line that was perfectly sea-level true, but wasn’t parallel to the floor, it would look wrong anyway. Right?
Standard advice on placing chair rail is about 32 inches from the floor, or alternately 1/3 the height of the wall. You could also go old-school and put it where your chairs’ backs actually strike the wall.
My fourth big tip: Tape a line on the wall first and eyeball it. If the height you’ve chosen looks strange, it’s easier to move tape than to repaint.
Sneakiest of all, though, was how I got around only having two hands. I bought little poplar planks and sawed them off at uniform height. Then I taped them to the wall I was working on (see above). Apply a little glue to the rail and hoist it up onto the slats; the slats prop it up level long enough for you to smack in the first few nails. Voila, spare hands!
my Aggie Journalists blog? It's over 
I'm not doing these right now because my life kind of went upside down, but thank you guys for the nice comments. I left all the stuff up 