Here is a tutorial to do that and have beautiful sewn mitered corners for the binding too.
Start by trimming the batting (carefully, to not cut the backing) to the edge of your quilt top.
I usually leave my edge basting in until the binding is done, but you could take it out at this point too.
Trim the backing to about 1" from the edge of the quilt top.
Fold the quilt on the diagonal at one corner as illustrated below.
Trim the corner (right angle to the fold) to minimize the bulk.
When you turn it in...the miter is sewn down and when it turns under, it encourages the fold down on both sides along the edge.