Sun sails are a great way to provide a minimally invasive area of shade to your yard. For these sails to be safe you need to be install them in a secure and sturdy way. Vertical sun sails often catch the wind and horizontal shades offer little aesthetic appeal. Thus the best way to install a sun sail is in a three- or four-post combination with two different heights of securing poles.
Lay out your sail on the ground where you want it to be positioned.
Measure out 1 foot diagonally from each corner of the sail and mark the spot with a stake.
Dig out a hole 5 inches in diameter and at least 6 inches below the frost line for your area, using the stakes as the centerpoint of the hole.
Fill in the bottom 3/4 of an inch of each hole with gravel.
Insert one post into a hole. Level it using the level. Pour in cement mixture.
Insert another full pole into the hole diagonal from the post already leveled and cemented If you are planning a triangular sail use the hole closest to the one you have leveled. Level and cement this pole as well.
Cut 3 1/2 feet from the remaining posts with the circular saw. If you are using metal posts you will need a carbine blade for this step.
Install both of these posts in the same way you did the first two. Make sure they are level before you cement them in.
Let the posts sit overnight before attaching any anchors or attempting to put up the sail.