Start by looking around and decide how to section the roof. (See plan below). The red dotted lines divide the sections. Count how many sections you have. Write this number down. Start with the largest section.
All your measurements will be, length times height. In the sample plan below all the blue arrows labeled L are the length. The red arrows are the height. The red arrow on the on the largest section of the plan below goes all the way from the front eave line, over the ridge and down the back, to the back eave line. Measure each section.
In the sample below you will have 5 different calculations and that should match the number of sections you wrote down, when you started. Add all your measurements together to get your total square footage. Measure all your hips, valleys, ridges, eaves and rakes. Divide your square footage by 100 to get how many squares of roofing material you will need.