They use the wind to make the initial supporting thread. They release an intial thin sticky thread which is then blown by the wind until the end sticks somewhere (either starting from your bushes and blowing towards your shed or vice versa). Sometimes they do this twice (2nd thread starting a bit lower. Then they move along the first (usuaslly only) line carefully, reinforcing it as they go. They may go back again and reinforce it anew until it is strong enough to support the net (to be built). They then proceed to attach a Y-shaped thread (actally at first it is another thread fixed first to one point along the supporting line, given a lot of slack while the spider moves along and then fixed again, hanging down loosely as a loop. They can move down this, their weight tightening the loop into a form of triangle). At bottom they let off another thread which the wind will stick somewhere, go along and reinforce it. Foundations laid, time to build the web. They can fix it with as many strands as deemed necessary by moving along the initial support line to its end, emitting thread as they do, climbing lower, attaching it (bush or shed), reinforcing it as above and then returning to the job.