Make a loop in the leadcore,about 4 inches from the tag end.

Next step is to slide back the outer coating on your leadcore,approx 3 inches.
Cut the lead off,then slide the outer sheath back out till it's snug.

Insert your mono leader(I prefer 20 lb),so it butts up with the lead inside,then work the knot down towards the tag end(keep the mono butted up again the lead inside).

Pull the knot tight,holding the dacron on each side of the knot(then snug it up tightly,holding the mono/leadcore to really cinch it down.

The knot looks like it will slip,but it doesn't.
The Willis Knot

Here's 2 links to leadcore trolling depths.....
Click Here
Oak Board Core Depths