there are some general guidelines to follow that make cutting ur hair a lil easier. I personally follow these and then once the basic shape/style is made I fuck around and shape it to my liking. Best way to go about it in my experience. A good general rule of thumb is to cut at longer lengths than you want and then shave/cut it shorter as you go so you can adjust it, that way you don't cut off too much and your stuck with something not quite what you want until it grows out. Luckily with such a layered/shaggy cut it's a lot more forgiving in terms of mistakes than other styles. Good luck!