Flange is a narrow fabric strip sewn between two adjacent fabrics. In this picture above it's shown in red. Piping is similar and often confused for flange but its stuffed with a cord or yarn. Flange is flat. This strip measures 3/4" wide and the length is as long as the perimeter of where it will be attached.
The 3/4 " flange strip is ironed in half on the long edge with wrong sides together. The raw edge of the flange is lined up with the raw edge of the quilt top and pinned in place, only one side at a time. The the second border, the adjacent border is added and lined up in a little sandwich with flange in the middle.
In the second picture you see from the underside of the quilt top how it looks and how the three layers line up. All the raw edges are lined up and are sewn together on the 1/4 seam allowance. The little extra piece of the flange fabric will pop out from between the adjacent border fabrics.
Stop sewing at the corners and begin the pinning process again down the next long side.
Try flange on your next quilt top for a little pop of color!