A sprinkler system is designed to transport water to a variety of locations in a short period of time, most commonly for landscape and lawn maintenance use. The systems are created with a series of water lines -- small branches called connectors or swing joints -- and heads that distribute the water. Any type of sprinkler system will need flushing at some point as part of regular maintenance.
Turn the irrigation system on and walk the entire system, checking each head for proper function. Mark any that are not working or are reacting slowly with a small flag or stick. You can also do a quick test to see if the problem is a small piece of dirt by stepping on the riser while the head is working. This causes it to quickly reactivate and the water pressure may release the dirt causing it to stick. Do this a few times, and if the head is still not working properly, mark it.
Turn the irrigation system off, and turn off the water valve to the system. If at this time all sprinkler heads are functioning properly and there is nothing to be repaired, you can simply remove the last head from the line, turn on the water and let it run for 10 minutes. Turn it off, return the last head to the line, and your flush is complete.
Remove the nozzles and screens on the spray heads you noted weren't working properly and install flush caps in their place. These come with the heads when the system is installed. If you do not have flush caps you can remove the heads from the nipples or swing arms by digging them out down to the connecting joint or swing arm. Do so carefully, making sure the top of the swing arm is exposed and the soil line is below the top so no dirt gets inside.
Turn on the water and flush the system for about 10 minutes. While the system is flushing take any heads you have removed and wash them by hand with a hose or faucet. Unscrew the casing and check the heads for dirt and debris.
Turn the water off and return any sprinkler heads to the system and reinstall them. Turn the system on again and walk it, checking for proper function of all the sprinkler heads. Any that remain in non-working order will need to be replaced.