I drew a picture for you. (The 2 year old and the four year old really wanted me to say "be nice" and they wanted to draw a couple hearts. So, that's what that's about.)
In the picture, the black would be a zone with with three heads set up in a series. The red is the added loop with additional sprinklers on the other side to demonstrate doubling coverage, but those aren't required. I also drew in red the original sprinklers newly gained throw coverage.
To just tell you the time, and not how the clock is made: Just put another splitter in between your first head and the faucet. Then run another hose from the end of the original line back to the splitter. At some point you're gonna have to connect the ends. I'd just keep it simple and cut the female off and put a male hose mender on it.
There's much more math and such involved, but this is a basic way of understanding how the clock is made: Notice how the heads closer to the source throw the water further? Water and air take the path of least resistance in a ducted/piped system. And at each head in a series is another opportunity for the water to take one of these paths. This leaves that much less water/pressure for the next head.
When you loop it back to the source, you're putting much more volume into the system. And creating an additional path back to the source essentially. After a period of time the loop will pressurize. Then every head on the system gets a somewhat equal amount of water pressure similar to what they'd have gotten connected to the faucet by themselves.
The picture on the right is just something showing a a little more what I meant by doubling your coverage from a top down view.