This does depend on what you have been mixing with the paddle, but as a general rule water is the weapon of choice. This is ideal for paint, grout, filler, cement and plaster.
Fill a bucket with water, place the paddle mixer in the water and switch on the drill. Stop and start to help remove any material until the job is done. When finished, wipe down with a dry cloth and store.
If you have used a paddle mixer to mix an oil based paint water in a bucket will not work. You can fill a bucket with a white spirit like this Tetrion White Spirit from Amazon, but this can be an expensive option.
A cheaper option is to dip a clean cloth in White Spirit and wipe down the paddle. You can finish off by filling a bucket with hot water and a squirt of washing up liquid and rinsing the paddle mixer in the water