Three things. First, you want to look at the grain of the fabric. Sometimes the grain will get pulled in one way or te other and the threads will not be perfectly perpendicular to each other. I was taught in fashion school to stretch any fabric until the threads are absolutely straight. If the threads are skewed not only will your fabric hang a little wrong, it makes it hard to cut straight.
Secont, use an L shaped ruler. After you draw your first line, place one of the edges of the ruler directly on that line while drawing your next line. Every subsequent line should be done this way, with a previous line being used as your guide. You can also use the 90 degree angle of your L ruler to make sure every angle is exactly 90 degrees.
Third, to be extra sure, measure the length of each line before cutting. Opposite sides should measure exactly the same. If they’re wrong, you can fix it.