I apologize for taking so long to respond. Attached are pictures of my jig and an application on a Great Lakes 1812-1815 circa Warship.
Thread goes into the spring at top with spacing required depending on diameter of the mast. Bottom piece where the screw goes into the jig is wood and slots are cut according to the spacing required on the hull. Start at the top with a piece of thread, then slip it into the spring, and then pull down into a location on the bottom piece of the wood both sides. The beauty of the wood is you can pick number of shrouds/lines, spacing etc. Then start glueing your lines in place according to the scale required.
Jig is built from brass square tubing. Holes are drilled to allow for different SIB scales. Just remove the brass screw each side and adjust accordingly. Rod is soldered at the bottom so it can fit into my fly tying pin vice.
Here is a picture of the rat lines installed on a British Warship.