It depends on the quilt & at what point you're trying to square it up. If it's still a top and you're squaring it to add a border--you measure through the center vertically & horizontally and cut & fit the borders to "square." If it's all ready quilted and you're trimming it to square before adding the binding--I use a carpenters square. If it is all ready finished, you have to get it wet and "block" it out like a sweater and that will only work if it's not too wonky. If it's really warped--the animal shelter will love it or keep it as an emergency/picnic quilt in the trunk of the car.