Measure the distance between the two points where the ends of the hammock are hanging from.
Purchase the replacement hammock cloth. If the hammock has a spreader bar, the length of the entire hammock between the two points it hangs from must match the distance between the two hanging points because hammocks with spreader bars are designed to be completely taut when not in use. If the hammock doesn't have a spreader bar, add 2 feet to the distance between the hanging points, as this type of hammock is supposed to sag when not in use.
Remove the old hammock cloth from the points that it's hanging from. Usually this involves removing the ring from the hammock from the hook or eyebolt that it's attached to, but this can vary depending on how you have installed the hammock.
Attach the new hammock cloth onto the hanging points.