Historically, duck was prepared in this manner to preserve the meat for use at a later date.
Rub the salt and spice mix into the duck legs, and then place them into a non-reactive dish into which they only just fit, sprinkling any left remaining salt over the top. Cover and place in the refrigerator overnight.
The next day, scrape off as much of the salt mix as you can and then rinse the duck under cold running water. Pat dry with paper towel.
Place the duck fat into a pot that is just large enough to accommodate the duck and melt over a medium/low heat. When melted, add the duck and then reduce the heat to approx 90˚C. At this point you can either place it in the oven, or cook it on top of the stove. (I prefer to cook my confit on the stove top, so I can keep the oven free for other use.)
Cook the duck in the fat for minimum of 4 hours. (I tend to cook mine for approximately 5-6 hours.) When done, it will be very tender and almost falling off the bone.
Place the jar of confit in the refrigerator to set the fat, and store there until ready to use. (For alternative method of storage see notes below.)
Serve with a Port and Fig Vinegar Jus (see notes) and salad of mixed leaves lightly coated in a dressing of Dijon mustard, red wine vinegar, olive oil, salt, pepper and a little sugar, whisked together to blend.
The duck fat should never bubble or boil. It should barely be simmering.
I prefer to remove the duck from the fat, once it has cooled, and then vac pac and freeze.
You could also serve the confit on a bed of mashed potato with the Jus, or alternatively remove the meat from the bones and toss through the salad and add some orange segments.
Link for Port and Fig Vinegar Jus