It's important to remember when doing a corner to stop sewing at the same distance as your seam allowance. If you are using 1/4" seams then you stop sewing 1/4" from the edge of the corner. I usually use a 3/8" seam allowance when sewing on binding because that is the width of my walking foot. In my case I stop 3/8" from the edge. If I stop 1/4" it won't fold correctly. If you use 1/2" seam allowance you stop 1/2" from the edge.
If you have trouble getting the correct angle. after you stitch to the proper distance from the corner take the quilt and lay it on a flat surface. Lay a ruler along the right edge of the quilt and fold your binding up to lay straight against the ruler. This will give you a perfect angle. Once you have that then fold the binding down against the ruler with the folded edge against the top of the quilt. This technique also works for corners other than 90 degree with a 45 degree angle.