Measure the distance between the two trees or between a tree and another fixed support. Many hammocks are designed for a maximum spread of 15 feet between the connection points. This will determine whether you can install the hammock on any particular pair of trees or other combination of supports.
Mark a spot on the tree where the hammock will attach, ensuring the attachment point is at least 5 1/2 feet off the ground. Depending on the construction of your particular hammock, you may want to mark the spot higher up on the tree to allow for the sag in the hammock ropes over time.
Drill a hole laterally through the tree trunk using a drill bit with a diameter large enough to accommodate the eyebolt. Extra-long drill bits are available from hardware stores.
Twist the eyebolt through the hole in the tree, with the eye of the bolt on the side where you want to hang the hammock.
Twist the nut onto the threaded end of the eyebolt where it emerges on the other side of the tree. Tighten with pliers.
Slip the steel hook on one end of the hammock onto the eyebolt.
Repeat the installation for the second tree or other support to hang the hammock at the other end.