A water heater flush removes the sediment that settles in the bottom of the tank. Whether you do it yourself or hire it out depends less on skill than on what your unit allows. Walk the checks below in order. If one of them fails, the usual answer is to hire it out.
Start with the manual and the install year
Find the manual or the model plate. The manual gives the recommended flush interval and says whether the manufacturer expects a drain at all. Note the install year. A unit that has never been drained is a different job from one that has been drained at each service interval, and the first run of water will show you which one you have.
Check the shutoffs before you touch the drain
An electric unit needs its breaker off before you start. A gas unit needs its gas control set to the off position as the label directs. Then find the cold water inlet valve on top of the tank and close it. An older gate valve may not close fully or may leak once turned, so test it before you rely on it. If it will not close, stop and call a plumber.
Check the drain valve and the outlet path
The drain valve sits near the bottom of the tank, usually with a threaded outlet for a garden hose. Look at it. A valve that is crusted, cracked, or seized is a reason to hire it out. Do not force it. The hose needs a route that runs downhill to a floor drain or outdoors, and it must carry the water without spilling. The water comes out hot, so plan the route before you open anything.
Check the space around the tank
If you cannot reach the drain valve with a hose and a bucket, or if you would have to crawl under stairs or work from a ladder, the job goes better with a second person or a trade. Look at the pressure relief valve and its discharge pipe. Leave that valve alone beyond looking at it.
When to do it yourself
Do it yourself when the unit has a working drain valve, the power or gas shutoff is clear and easy to reach, the water path runs to a drain, and you are comfortable disconnecting and reconnecting the cold inlet. Keep the power off until the tank is full again. Open a hot tap elsewhere in the house to purge air before you restore power.
When to hire it out
Hire it out when the drain valve will not open or close cleanly, when the unit sits in a tight space, when the job reaches past closing a valve into the gas control or a gas line, when the water stays so thick with sediment that it will not run clear, or when you have no clear way to shut off the power or gas. Ask the trade to write down what they flushed, what they found on the valve, and the next recommended date.
Questions to ask before you choose
Ask whether the drain valve was tested. Ask whether the cold inlet valve was tested. Ask whether the trade will check the pressure relief valve and tell you its condition, and whether they will replace it on the same visit or only with your agreement. Ask what happens if the drain valve fails during the job. The answers tell you whether the job stays simple.