Cover the bottom of a non reactive container that's just large enough to accommodate your duck breasts with about 1 inch of coarse salt, then and gently lay the duck breasts flesh side down on top of it. Make sure that the breasts don't touch the sides of the container.
Pour enough salt over the breasts to completely cover them and then place the container in the fridge, uncovered, for 12 to 24 hours. The salt will cure the meat and start to draw the moisture out of it. The longer you leave it in the salt, the saltier the final product will be.
After that time, remove the duck from the fridge and quickly rinse it under cold running water.
Pat it real dry and then sprinkle the flesh side with coarse ground black pepper. Place a few sprigs of fresh thyme and fresh rosemary (or other herbs of your choice) over each breast and then close them together like a sandwich, flesh to flesh.
Tie the two breasts together tightly with butcher's twine and then wrap them loosely in a clean tea towel. Place that package in a shallow plate, ideally one that will allow air to circulate all around. Place your duck in the back of the fridge, where you will forget it for the next 2 to 3 weeks. You can still check on it from time to time to make sure everything is going fine! The longer you leave the meat to age, the drier it will get.
After that time, remove the breasts from the fridge, untie them and discard herbs, then slice thinly and serve with crackers (or use in sandwiches, salads, pizzas, or as a side with eggs)
Store in the refrigerator in an airtight container for up to several days.