Why make a jack line?  If you single sail like I do, the thought will eventually hit you, "What would this boat do if I fell overboard right now?"  I know those things never happen but with a little nylon webbing and some sewing you could be attached to the boat.

What you need

1. 37' Tubular Nylon Webbing - 3,600-lb breaking strength

2. Thread (UV-69) - Canvas Shop - Samples, $0.50/ea

3. One 4" SS Carabiner - West Marine ($12.00)

Sew the Carabiner on the Bow end

Measure the length out to fit your need.  I wanted my line tight and in a straight line to the stern.
     Loop the webbing through the carabiner and overlap the webbing about 6".  Sew a straight line of stitches top and bottom of the overlap and an "x" in the middle.

Sew the Loop at the Stern end
 

On the other end you need to just sew a loop with a minimum of 9" of opening space. This will provide enough loop to connect to any cleat in the stern.

Sew the end the say way as the opposite end.

Problems
     While out sailing I connected up my jackline, safety harness and tether.  What I noticed, during most favorable conditions (in contrast to a very hard wind and ruff water) was that my tether was catching on anything and everything it could get caught on.