The gutters should be cleaned at least once a year and more than that if you have trees close to the house. Cleaning the gutters is an important preventative measure so that your foundation doesn't get soaked and siding and yard don't get ruined in a storm due to misdirected water. It's important to make sure you have gutters in all of the appropriate locations such as at roof slopes and around the entire home.
Try to choose a calm, non-stormy or windy day. Make sure to wear a dust mask, safety goggles, long sleeves, slip resistant shoes and leak-proof gloves if possible. Use an extendable ladder and make sure that it's stabilized using a ladder stabilizer or stand-off. (When using the ladder, make sure that you always have at least two feet and one hand or one foot and two hands on the ladder at all times. It's easy to get distracted while working so be sure to follow this protocol.)
Place a tarp nearby to drop the debris onto. You could use a plastic scoop, but using a hose with a high-pressure nozzle for a clean scrub would be best. Depending on the type and size of debris, you may need to use both. Make sure to rinse toward the downspout. You can use a wooden paint stirrer for any tough to move debris and for scraping material off the bottom of the gutters.
Once completed, make sure to do a thorough rinsing of all gutters to check for any leaks. Mark any leaks with a grease pencil, so you can patch and/or correct them when it's dry.
If you don't have gutter covers such as mesh screens, clip-on grates, installing them could help, but it will make cleaning the gutters more difficult.