You can look on you tube to see how to use a protractor. The problem is going to be how to keep the angle
true on a long cut. Just a small miscalculation at the narrow end is going to result in a large miscut at the long end. An adjustable protractor is going to be your best bet. These are often used for woodworking, so ask around a friend or neighbor may have one. Cut a block using muslin first before using good fabric to 'proof' the angle.