Cleaning a blender isn't a really easy task. Shoving your hands inside and ensuring that you don't get cut from the blade inside requires a lot of precision and can only be achieved if you start thinking smart. Anyone who has ever cleaned a blender before would agree that the correct way lies in letting the blender do the work itself, rather than you doing the tedious scrubbing.
All you need to do here is to fill your blender with warm water. When you have filled the blender with warm water, wait for a couple of minutes before adding two to three drops of dish soap. Once the mixture is created, you can turn the blender on and can have the device rev up, while cleaning the mixture inside it as well. This will do the job for you.