This is a video showing how to measure, mark, chalk lines, and install a standard three tab shingle roof. The same approach works just as well for a garage or a house roof too.
It would be great if you go post a comment to the second part of this 2 part video when you've done your roof and can actually comment on how well it worked for you. I'm always open to constructive criticism too; I do good sometimes and not as good other times, like anyone else. The second part of this video seems to draw in some guys who think only their way (probably their bosses way actually) is correct, though, for some reason. I'm probably more open to new ways of doing things than a lot of people are, though, mainly because I get into situations where I work alone much of the time and can do things any way I want to do them whenever I see a way for improvement to the normal way of doing things.