How do you install a sloped roof over an extisting flat roof?

You cut rafters, brace them as needed depending on the span and shingle it. I put one on a friend of mines. I cut the outside ends of the rafters at an angle so that I didn't need to change the fascia. Lay a 2x6 flat on the existing roof to run the braces down to. Your roof may not be flat but have a slope one way on it so you will have to take that into consideration for the peaked roof. He had a slightly sloped roof with tar and gravel on it and at the time I was able to put a pitched roof on for less than they wanted to re tar the flat one.