Tidy up the duck legs by pulling off any large bits of fat and trimming any skin that hangs way beyond the meat. (You can put the skin and fat in a small saucepan over low heat to render the fat; save this for confit or for another use.)

Place the salt, garlic and sugar in blender or mortar and blend. Add the peppercorns roughly chopped.

Holding one duck leg at a time over the bowl, rub a generous amount of the salt-sugar mixture all over the leg, into the skin and flesh. Repeat with the remaining legs. In the bowl or another container, pack the salted legs on top of each other, layering them with the peppercorns, and thyme. Sprinkle with any remaining salt mixture. This step should not be rushed.

The next day, unpack the duck legs and rub off any salt and spices with brush. Pat dry. 

Melt the fat or lard in a wide heavy-bottomed pot just big enough to hold the legs. Add the duck to the fat; it should be submerged. 

Simmer the duck very slowly until the meat browns, shrinks off the bone, and is very tender when pricked with the point of a knife. wait 3 hours before checking for tenderness, and be prepared, depending on duck, to cook an additional hour or more.  (Be sure to check the temperature every 20 to 30 minutes to make sure it's not going too high.) fat should never go much above 200ºF during the cooking time. 

Remove the pot from the heat and let the duck cool in the fat to room temperature. When cool enough to handle, remove with tongs, taking care not to rip the skin, which will be delicate. Can either eat the duck as it or transfer it to a storage container, cover with the strained fat, and chill until ready to use.

To use, bring container to room temperature so that you can gently pry out the number of legs you need, scrape off the excess fat, and press the fat back over the remaining legs.