Right sides go together!
I lay the left side end along the unsewn edge and stick a pin in it to anchor it to the edge where it's not been sewn. Then I take the right side end and lap it over the left side end, and mark with a pin exactly on top of the left side pin. Don't pin it to anything, just put the pin in the binding as a marker.
Open both ends flat, moving pins if you have to but then putting them back in their exact spots again. Lift the right side end up so it is perpendicular to the left end, and lay it with its pin right over the pin in the left end. Use both pins to join the ends, right sides together-- the left side parallel to the edge and the right side perpendicular to the edge. Then sew a diagonal seam from the upper right corner, down to the lower left corner, where the binding ends overlap. Trim to a 1/4" seam allowance and your binding should be the right length. Sew it on.
That, without pictures or a video, was probably about as clear as mud! Hope you can find a video of the process.