HI Helen, one way i know of (there maybe more/different ways)... on one side of the quilt has the seams look like regular seams sewn as usual. the other side has all the frayed/raggy seams.
to get this make 2 sets... lay one 'front' (raggy side) square and one 'back' (regular seams) square, wrong sides together.
lay these two 'sets' with the 'back' fabric right sides together. now sew the seam and open. size of seams is up to you. tho, iirc, about 1/2" up to 3/4" seems about right. add seam allowances to finished sq size accordingly when you cut the starting squares.
you should have a clean seam on one side and an exposed seam on the other side. once you've sewn the whole quilt together and go back and (using 'spring to open' scissors, to make it easier on your wrist/hands) snip along all the seams about 1/8 to 1/4 inch apart.
they will fray when its washed so it is "HIGHLY ADVISABLE" to wash the quilt inside a big cloth bag. saves the mess/damage to your washing machine. i do hope that is clear. as i said, i'm sure there are other ways to do this, thats the one i know. cheers, jeanne