Grind the black and white peppercorns in a spice grinder to a very fine powder. Add to the pork along with the pork skin, minced garlic, sliced chiles, sugar, and salt. Mix well with a clean gloved hand or spatula.
Add the nem chua powder (don't forget the contents of the small inner packet). With a gloved hand, knead the mixture by grabbing handfuls, pressing firmly into your palm, and repeating until the mixture is sticky and paste-like, about 5 minutes.
Line a 20 cm square baking dish with plastic wrap. Transfer the mixture to the pan and smooth into an even layer with a spatula. Place 1 slice of raw garlic and 1 chile ring on the surface about every 5 cm.
Cover with plastic wrap, pressing it firmly onto the surface, and refrigerate for at least 24 hours and up to 1 week before cutting and serving.