First, it saves you the time and trouble of having to tend sprinklers and drag hoses around the yard.
Second, by matching the watering needs of your landscape to your schedule, climate, and plantings, it saves you money that would be wasted by inefficient manual watering. Your savings will increase even more if you've been leaving soakers and sprinklers on all night.
Installing an irrigation system requires some manual labor, but it's not backbreaking stuff, especially if you don't rush it. You probably can't get the whole thing done in one weekend, so you might as well take your time. Furthermore, modern technology has made assembly of the system itself -- the valves, fittings, piping, and spray heads -- even easier.
There's one aspect, however, that you won't be able to avoid, and that's planning your system.
Planning starts when you begin to map your irrigation system -- making a careful sketch of your yard, designating trees, shrubs, sidewalks, fences, slopes, sunny and shaded areas, and all permanent structures and planting beds, along with dimensions between them and a brief description of what's planted where. That information will help your supplier determine what kind of sprinkler head will meet the specific watering needs of each area. Your map will also need to show where the manifold (connection/distribution point ) will be, usually in an inconspicuous spot close to the water line.
Your supplier also will need to know something about the water pressure at your house, and most will lend you a gauge to measure it. Water pressure and flow rate matter because you can't have more than 60 to 75 percent of the total pressure going into an irrigation system. If you need more heads than this general rule allows, you can establish subsystems, each with their own dedicated valve, programmed so they water at different times.
When it comes to timers, you'll find that manual timers, though inexpensive, require more attention than you may want to give to a system you intended to do the work for you. Automatic timers irrigate your landscape right on schedule, even when you're not there.
When you begin installation, your sketch will prove invaluable. You'll still need to lay down mason's lines to guide your excavation, but your irrigation map will show you where to start and stop and how many heads to install along a given line and at what intervals.
You'll notice that each sprinkler head is designed for a specific watering need. Many are adjustable to allow targeted spot-watering without watering objects such as sidewalks, driveways, or the house.
Lay out your entire system with stakes and mason's lines. Use the lines as a guide or mark the ground with paint and move the lines out of your way. Excavate to a depth of 8 to10 inches (but consult your retailer for depth in a cold climate). Save the sod to re-cover the trench.
Install a control valve for each circuit in the system. Some types are made to be wired to the timer. Manual control valves are less expensive but far less convenient. If budget allows, get the low-voltage models. Make sure the covers of all the valve boxes are at ground level.
Lay out the pieces on the ground in the order they will be installed. Cut pipe to length and dry-assemble the pieces outside of the trench in sections. Lay a section in the trench to make sure it fits. Then disassemble, glue (or clamp), and reassemble. Mark spray head locations clearly.