I hope I can get this explained well enough for you to understand. And HOPE this is the look you were wanting.

1. do a row of tumbling blocks to mirror your last row.
2. sew that row onto your quilt top.. right sides together.
3. turn and press, do clip the edges where needed to make it all lay flat. Makes a finished edge all around your quilt..or on just 3 sides.. YOU choose.
4. Mark the center line of your tumbler blocks.. where you will attach the next row.
5. Sew your border onto the "center" line of your tumbler.. with both fabrics facing up.

This will leave a flap of fabric that is finished off where you join your border to the quilt top. If you wish to tack it down or just leave it as is.. that is another option.