Take a measurement of each side of your quilt. Then make a 6-8" strip of paper (newspaper, typing paper, or whatever) the same as these measurements (one each). Fold and fold and fold until you have about 5 - 9" spaces. Depends on the side scallop you want. Using a plate (dinner, salad or saucer) and draw your arcs to fit inside the folded spaces. Remember that you will have a "half plate" at each end which make almost a "full plate" at the corners. Do the same with both sides of your quilt. This will create your patterns. Trim off excess paper and pin to the quilt and carefully cut your quilt.
OR:::: Mark your quilt around your pattern, and at this point, I think I would carefully stitch just inside these marks. Then trim. This way you should have no stretching and it will also compact the edge you will binding. Cut on the drawn line.
When attaching your single- thickness bias binding (it must be bias and it must be narrow - no more than 1/4" or 3/8" **) by machine, once you get into the cleft of the arc, stop, turn the quilt 1/4 turn, take one stitch, stop, turn the quilt 1/4 turn and continue stitching. This one stitch (which will be parallel to the body of the quilt) in the cleft will allow the binding to lay nicer when you turn it to the back and will be much easier to hand stitch.
** Cut your strips 1-1/8" if your batting is thin. If fatter, cut 1-1/4" strips and be sure to cut and sew your strips together at an angle to distribute bulk.
Hope this is clear and helps. Suzanne