As a rule, don't. The discs will be self cleaning in all but the very worst condition. Cleaning increases the possibility of contamination with oils that ruin the braking performance.
If you do decide to clean them, you must be pedantic about preventing oils and other contaminants getting on the rotors and pads. A simple wash with a hose, keeping direct pressure away from the calipers, should be all that is needed.
If you want to clean the disk further, use a clean cloth or paper towel. Isoprop can be used but should not be needed.
To clean the outside of the caliper, wipe down with a clean, wet cloth. If the caliper is clogged up, I would remove the wheel and disk pads and gently take to it with just water and a slim brush (The ones you get to clean straws or tubes of drinking bladders are good).
Wash the pads in water, using a clean brush or cloth to remove excessive dirt then reinstall them.
Be cautious of soaps and other cleaning fluids as these can leave contamination. Use a plain detergent.
Keep the compressed air away from the brake calipers.