If you cut up a duck for its breasts for dinner, put the rest of the duck (legs, backs, wings, necks and giblets - everything except the skin) in a pan. Season well with salt, pepper, thyme and onion slices, cover and put in the refrigerator. It will need to marinate for up to 24 hours.
To get most of the fat needed to start this process, put the skin you removed from the breast in a glass container and heat it in the microwave for 15 minutes, covered. You should get about a half-cup per breast skin. Drain through cheesecloth and set aside.
The next afternoon or evening, brush off any remaining salt and put the duck in a crockpot. Heat the fat gently and pour over the duck. Turn the crockpot on low. After about an hour, if the duck isn't submerged in fat, add olive oil to cover. Check again in about three hours. If the wings get done first, remove them, cover and set aside. When the meat is all done, remove it from the fat and cool. When the fat cools a little, strain it through cheese cloth. You can skim the fat off the top from the liquid after it is refrigerated and use it for another confit.
Take the meat off the bones. You then have three options: Eat immediately, either as is, or crisped under the broiler; can the duck; or cure in a crock in its fat.