It's not necessary to know how to use a framing square or what the pitch is. As long as you are not working from a blueprint everything is up to you. Simply hold the rafter up until the pitch looks right. Then nail a board against the back of the building to hold the board at that height. Now take a level and draw a plumb line at the top this will be your angle for the ridge (when cutting move the line down 3/4 of an inch to compensate for 1/2 the thickness of your ridge board). Next, go down to where the rafter hits the wall and trace where the top plate meets the rafter this will be your birds-mouth cut. You do the tracing on the end of the building so that you can use the top plate to trace against. If you are not familiar with the birds mouth or ridge look them up. If you are going to have a fascia then let the rafter overhang and use the level to determine the angle to cut in order to have it straight up and down. As far as using cedar for the rafters, if it's the weight that you are concerned about you shouldn't be. One person can easily handle a rafter. all you do is set it on the outside wall and lift it up and nail it to the ridge board.