A lot of it depends if you want to make a mitered border (one that looks like a picture frame) or a straight border.
For the straight border you measure the quilt down the CENTER and cut your first border to that dimension. Pin and sew to each side. If there is a slight difference between the middle and the edges, you can distribute the difference (either easing or slightly stretching) over the entire length. For small wallhanging, you shouldn't have that problem. Press the borders.
Now measure down the center of the quilt in the other direction and repeat the steps.
I would recommend that type of border to start.