I do it the same way I've observed barbers doing it when I got haircuts. Grab a swath of hair between middle and index fingers, pull fingers up until desired l

I do it the same way I've observed barbers doing it when I got haircuts. Grab a swath of hair between middle and index fingers, pull fingers up until desired length to cut off is sticking out, cut using fingers as a guide. I have really fine hair though so slightly uneven or layered cuts look better and give my hair more body and it's actually better if it's not exactly perfect.