Depends on the printer brand/model. Some have easily removable heads, some don't. For the first category just revove the heads and soak the nozzles (NOT the entire head) in a bit of alcohol. For the second category, soak a paper towel with alcohol and press it firmly into the nozzle surface, and let it sit for a few hours.
But clogs can occur as often or more often elsewhere in the ink supply chain, such as where the supply tubes feed into the cartridges, or even entire portions of the tubes can get clogged if air is introduced for long periods of time.