Thats a frequently asked question so far. Mmmh, we have to put this on our to do list.
So I try to describe what you have to do in single steps.
1) fix one end (loop) of each line on a durable hook. (doorknob, ground-stake)
2) unwind the whole line
3) stretch it with some good pull of the weight of you body to get any linestrech out of it.(be careful that nobody opens the to where you have attached the line ;.)
4) open the knots of the loop in your hands. Maybe do that before stretching the lines, so it will be easier to open the knots.
5) be careful to not let the sleeving come of. Otherwise you need a sleeving kit.
6) bring both lines to the same tension by let the line sliding trough your fingers
7) make a mark an few cm in front of the sleeving. That mark should be on the same high by pulling both lines with the same force.
8 move the sleeving up to the and keep it there for all next steps
9) lengthen the sleeving so that it fits tight to the core.
10) make a knot on the end of the sleeving so that it can't be slip of the core.
11) Fold the sleeving by half, so that both ends of the sleeving come together.
12) secure this loop by on simple overhead knot and check the linelength again
14) make a second overhand knot, or a figure eight knot in the sleeving.
A figure eight knot has the advantage to reduce the braking strength of the line not as much as an overhand knot. But it's a bit harder to do, and it's harder to open it again, if you're not lucky with the length of the line.
I hope the helps a bit.