Heat the milk in a large pot over medium heat until it reaches 185°F (85°C). Stir occasionally to prevent scorching.
Remove from heat and let the milk cool to 110°F (43°C).
Stir in the plain yogurt and skyr (or mesophilic starter culture) into the cooled milk.
Add the rennet and mix well.
Cover the pot with a lid or plastic wrap and let it sit in a warm place (ideally around 110°F or 43°C) for 6-12 hours, or until it has set into a thick yogurt-like consistency. You can use an oven with the light on, a yogurt maker, or a warm spot in your kitchen.
Once the skyr has set, gently ladle it into a cheesecloth-lined colander or strainer. Let it drain for 2-4 hours, or until it reaches your desired thickness.
Transfer the drained skyr into a bowl and stir in a pinch of salt, if desired.
Enjoy your meal!