Dice the onion reasonably small.
Put the onion into a microwave-safe bowl.
Add about half of your volume of onion with salt.
Use a spoon or spatula to mix the onions and salt.
Microwave for one minute, just until heated through.
Lay the cloth wrapping on a flat surface.
Pour the onion and salt mixture onto the cloth and spread it to about the size of the person's chest.
Wrap the mixture with the cloth so everything is contained in a nice package.
Have the patient lie on a flat surface, propped up slightly.
Oil their bare chest with olive oil.
Make sure the compress is not too hot to be on their skin.
Pick up the entire packet carefully and place it onto their chest.
Place a towel over the wrapped package.
Cover the towel with a heating pad or hot rice pad.