We have a free shade sail installation guide you can download which has a lot of information on what you need to do when considering installing a shade sail.
(pre-made) The most common mistake people make is installing there fixing points before they get their shade sail. To avoid any mistakes when installing a pre-made shade you need to get it first and lay it out on the ground with the fixing accessories you are going to use to erect it, such as turnbuckles (8mm 210-290mm) fully extended then put you fixing points in. If you don't allow enough room between the shade sail and fixing point, you may have trouble getting it to tension. Also, the shade sail needs to be tensioned along the bisecting line, if pulling the wrong way this can also cause the shade sail, not to tension.
(custom made) Custom made shade sail is made to fit perfectly to your already installed fixing points. Before you install any fixing points we can give you an estimate on how much a custom made shade sail will cost. The price between are custom made shade sail and a pre-made shade sail are very different.
Do it once, do it right!