Pour your milk into a large pot and place on the stove over medium heat. Place a candy thermometer in the milk so that you can watch the temperature. You want to slowly, steadily heat the milk to 185-190 degrees Farenheit. Avoid heating it too fast or it will scald. 

If it does hit the scalding point, just avoid scraping the bottom of the pot- the scalded milk should remain in the pan when you pour it off. If you notice that scalding is beginning, turn the burner down to low heat. 

If you are using a vanilla bean in your skyr, add that to the milk before heating as well. 

When the milk reaches 185-190 degrees, remove it from the heat and allow it to cool back to 110 degrees Farenheit. 

Remove one cup of your cooled milk to a small bowl or a liquid measuring cup. Thoroughly whisk in your starter culture (your plain Siggi's or other plain skyr). 

Return the mixture to the pot and stir it in to fully incorporate. 

Whisk your rennet into 1/4 cup water- lukewarm, room temperature or warm water should all work fine, but not hot or very cold.

Add your water & rennet back to the pot and stir briskly for one minute to fully incorporate. 

Put the lid on your pan, move it to a warm place in the kitchen (you want to avoid drafts or drastic temperature changes). Insulate your pot by draping a bath towel or blanket over it. 

Leave it for 12 hours.

When you remove the lid, you should be able to see that the skyr is visibly separating into curds and whey. The whey looks like a yellowish liquid floating around the top and sides.

Line a colander with a double layer of cheesecloth and place it inside a large mixing bowl.

Use a wooden spoon to scoop all of the curds out of the pot and onto your cheesecloth. Tie it in a knot and hoist the whole thing up so that the whey drains down into the mixing bowl. You need to hook it to something- I loop it over the handles on my cupboard.

Leave it for several hours- at least four. 

After the skyr is thoroughly drained, take it down. You can serve it right away if you like, or put it in an airtight container and transfer it to the fridge for later. 

Skyr will keep tightly covered in the fridge for a few weeks! Save at least a few tablespoons to culture your next batch of homemade skyr.