This is a double fold binding. It has already been attached to the front of the quilt. If you use a 2.5″ binding then trim from where it is stitched leaving 3/8″. If your binding is 2.25″ then leave 1/4″ seam allowance. This should give a fairly even amount of binding on both sides of the quilt.
The first photo shows the top side of the quilt with the binding sewn and folded at the corner.
The next photo shows the back of the quilt with the binding folded towards the back. It will bend upward a bit right at the corner.
Take one side of the binding and fold down and pin or attach binding clips to hold in place. The binding needs to lay flat right to the other edge of the quilt back. You can hand stitch this in place or go to the next step. The important thing to note is keeping that first folded edge all the way up into the opposite edge of the quilt back. Holding that in place you will fold the left side binding down over it and pin or clamp into place.
In the last photo I’ve got my scissors showing how the binding beneath has to be tucked all the way up under the edge that is folded over the top. Pin in place and hand stitch down. If the corner lays flatter with the opposite side tucked under go ahead and do it that way! Easy Peasy!!