Take a litre (or however much you want) of cream (cow) and heat in a double boiler to around 83C.- that's really hot. At this point add about 1/4 teaspoon of tartaric acid (mixed with a little cold cream) and stir slowly for a few minutes.
The cream should slowly thicken and you will begin to see small flecks of curd. If it doesn’t coagulate add a little more tartaric acid.
Once this has occurred remove from heat and let cool for a while. Then pour into a colander lined with a fine cloth (cheesecloth is too coarse) and sit over a bowl in fridge to drain overnight. You might need to scrape the bottom to make sure the draining process continues.
You can alter the consistency of the mascarpone by how much you drain it.