Measure the distance between the two end support posts and compare it to the stretched-out length of your hammock. When hammocks are new, they tend to be shorter than after the first week or so of use. If you have a new hammock, add about 4 inches to the difference in length.
Divide the answer from step 1 by two to get the lengths of the two sections of chain you need for both ends. For instance, if your hammock is 6 inches too short (after adding 4 inches if it's new), you need 10 inches of chain, or two sections that are 5 inches each. Don't exceed 18 extra inches on either end or the hammock will be too unstable.
Attach an S-hook to each end of the hammock support posts. Connect a section of chain to each S-hook, then attach the hammock end rings to the chains with additional S-hooks.
Adjust the tension of the hammock, if necessary, by slipping the S-hook through a different chain loop. The hammock should sit almost parallel to the ground, with only a slight dip in the center.