The easiest way to learn how to sew blanket binding is to use bias tape. Bias tape comes in lots of different colors and widths. You'll likely find it in the sewing section near the zippers and buttons.
To sew a blanket binding, pin bias tape all around edges placing
one side of the tape on one side of the material and the other
side of the bias tape on the back side of the material. Pin in
Make sure material on front and back is inside your bias tape. If
your material escapes out of the binding you'll need to take the
section of your seam out with a seam ripper. Repin it and sew it
Sew all along edges along the edge of the bias tape. Take your
pins out as you go and doublecheck to make sure material catches
inside the tape on both sides.