Trim fat from legs and thighs, leaving skin intact over meat, but removing excess. In a bowl, combine duck with salt, juniper berries, bay leaves, and garlic, and rub salt mixture all over the duck to cover completely. Cover and refrigerate at least 24 hours and up to 2 days.
Remove from refrigerator and rub off excess cure (reserve garlic). Melt duck fat over medium heat in a Dutch oven large enough to hold duck, with about 3 inches space at the top. Add duck, skin side down, and heat until fat reaches about 200 degrees (test with a candy thermometer or electronic probe.) The surface should look like it is gently boiling (but should not actually be at a boil). Adjust heat if necessary to keep temperature consistent throughout cooking. Cook until the fat is clear and a knife stuck into one of the legs slides out easily, about 3 hours.
Transfer the legs to several glass, stainless-steel, or glazed-stoneware containers. Strain fat, discarding any solids and pour, still warm, over legs, making sure they are completely covered. Cool to room temperature, then refrigerate until ready to use, up to 3 weeks.
Remove desired amount of confit from fat, scraping off any excess, and keeping remaining legs covered with fat. Place skin side down in a cold cast-iron skillet or other heavy skillet. Place over medium-low heat and cover. Cook until skin is crisp, spooning off excess fat as it cooks, about 10 minutes. Serve as desired.