Cut a square with sides about twice the width of the facing, and round one corner of the square so it will make a smooth curve joining the facing. The facing strips for the sides are cut length of side, minus width of two squares, plus one inch for seam allowance. One side strip is cut in two pieces to make the join. (On a large quilt you might want to make a join in the middle of every side.) You sew the strips to the squares, turn under and stitch a 1/4 inch hem, and attach the entire facing to the front of the quilt, right sides together. The final join is made by overlapping the two ends. I don't have a photo of this because it's under my hanging sleeve. Then turn the facing to the wrong side, pushing the bulk firmly into the corners and squaring them, and slip stitch the facing down, or fuse if you want. This makes a neat facing.
Here's the other method that I don't recommend, although perhaps it would work better with a bit more care than I used. I don't remember where I saw this, so I can't give credit or blame.
A facing strip is cut for each side, about an inch shorter than the quilt. Then cut four squares large enough to cover the facing when it's folded into a triangle as shown. Fold the squares in half, press them and pin a triangle to each corner of the quilt on right side. Pin the strips to the sides of the quilt on top of the triangles. Stitch around all sides, and turn to the right side. The folded triangle should cover the raw ends of the side strips. Turn under the edges of the strips and slip stitch or fuse them down. Slip stitch the triangles over them. This method is easy, because there are no joins and the measurements don't have to be so precise, but I think it's bulky, and I had trouble making a smooth square corner.
On a very small piece those triangles could be left un-sewn, forming a place to slip a hanging rod.