Let's start this section by pointing out that every automatic valve has a specific direction of flow. Indexing Valves have an inlet port on the side that's pretty obvious, and zone or exit ports on the bottom. Check the instructions for a diagram of the ports to be sure. Solenoid Valves have direction-of-flow arrow printed or stamped somewhere on the body of the valve (the bottom piece which has the inlet and outlet ports.) The direction of flow can also be determined by the location of the solenoid, which is usually on the outlet side of the valve. Caveat: it's possible that there's some obscure valve out there that's constructed differently, but I've never seen one. Manual valves generally don't have a recommended direction of flow.
The first decision to make about valves in the planning & design process is whether to use a Master Valve. A Master Valve is an additional solenoid valve that's located at the beginning of the irrigation main supply pipe. The main purpose of a Master Valve is to prevent catastrophic leaks and loss of water on the main pipe. The Master opens simultaneously with the zone valves, so it's closed during the time that the sprinkler system isn't operating. So the majority of the irrigation main isn't under constant pressure the majority of the time. During operation, the irrigation main will only be experiencing dynamic pressure, which should be less than static pressure. Most modern irrigation timers have the capability to run a Master Valve. There will be an extra wire terminal marked P (pump) or M/P (master valve or pump.) don't worry if it's a P terminal, the timer will be able to power either a Master Valve or a Pump Start Relay. The timer will have an output capacity of around 1,000 mA. Most irrigation valves require 350 – 450 mA inrush current to open, so 1,000 mA will be more than enough. Check the timer's manual or look on the wall-wart transformer to be sure.
It's usually best to cut the outlet side pipes for a replacement, and a union is great to install on the outlet side when using threaded valves.
It should be noted that manifold configurations usually use more pipe to get from the centralized location to each zone, but less wire. Bundled irrigation wires are acceptable to use, if the distance from the timer is within the specs for the smaller wire. A satellite configuration is where each zone valve is placed in the vicinity of the zone that it's supplying. This config uses less pipe but more wire. Larger single strand irrigation wire is typically used since longer wire runs are involved. Also, the irrigation main supply pipe will be longer for a satellite config. Larger properties and commercial installations usually satellite the valves. Each valve can be placed in a 7” or 10” round valve box, although I've seen two valves in a 10” round box (kinda tight in there.)
Order of Components
The order of components on the irrigation main is: supply point > backflow preventer > pressure reducing valve > master valve > zone valves. The pressure reducing valve is an optional piece that may be necessary if the supply pressure is too high. The master valve is optional as well, a safety feature to prevent loss of water from a main line leak. The other variation is when using Anti-Siphon Valves. These valves are a combination of solenoid valve and backflow preventer, so a central backflow preventer at the beginning of the system isn't necessary. Check your local codes, some cities or states prefer ASVs over Double Check Valve type backflow preventers.