Step 13: First steps 3 - plan your lengths
The instructions say to fix strips of fabric horizontally. That is what I did in the main roof space - though when it came to the eaves I made some vertical strips, as you'll see later. You can use the quilt vertically throughout if you prefer (along the rafters instead of across them) but you will have to deal with many more joints, which means more wasted fabric and loads more sealing tape.
Make sure you get your overlaps right. I found that the margin between the seam and the edge of the fabric was about right, making an overlap of about 8 inches. The manufacturers say a 3 inch overlap is enough - but I found the slippery quilt slides around a lot, and a 3" overlap soon becomes an 'oh, whoops, it doesn't quite reach'.
Make sure all joints of fabric are where you can get at them. I didn't.
Try not to make cuts longer than they have to be. A cut more than a foot or so is only going to waste fabric: you're better off making a slightly larger overlap at the other side.
Consider using short lengths: they are more manageable. On the other hand, joints are tiresome and waste fabric.
Start nailing from the top: let gravity stretch the fabric for you. Otherwise you're pulling against it. You will want to keep the fabric taut in two directions in any case - three is impossible.