This recipe assumes you use 4 liters (1 gallon) of milk. Traditionally, you're meant to use skim milk, but I prefer whole milk. This recipe does not use rennet. Measurements in both Metric and American Customary are written for your convenience. For ease of preparation, an Instant Pot works swimmingly for the boiling and inoculation.
• Milk • Skyr (to inoculate) • Cheesecloth
Boil milk at about 83°C (180°F) for 15-20 minutes.
Let it cool to 43°C (110°F)
Mix about 150-250ml (2/3 cup-1 cup) of starter with some of the warm milk and stir well until it's all even. Pour this back into the vat of milk. If you have made this recipe before, then you can, and I suggest you'd best, use the earlier batch's skyr instead of buying another from the store.
Culture at 43°C (110°F) for at least 12 hours. I prefer 16 hours myself. The longer it cultures, the stronger, tarter, perhaps even umamier its flavour shall be.
Spread the cheesecloth, preferably over a strainer or colander, over a large bowl (e.g. a tupperware bowl). Pour the vat of fermented milk upon the cheesecloth. I always put a lid or covering over it. Put it somewhere in the fridge, and let it strain for 48 hours (12 to 24 is OK if you're an impatient chump, but everybody knows, the thicker the better). Check in now and then to make sure the separated liquid whey, which should be at the bottom of the bowl, has not gathered so much as to back into the skyr. It may need to be emptied halfway through the straining process so that further whey may drain. I also strongly suggest saving the whey instead of casting it into your drain.
Whisk it up well to smoothen the texture. You may now jar it, and enjoy it.