Put the cream in your processor / blender and leave on a medium speed. It will whip up, get thicker and then turn into an overwhipped fluffy mess. Keep going, scraping down the sides if necessary. After about 15 minutes, very suddenly it will turn into a clumpy mass with a liquid sloshing about. You’ve now made butter, and the liquid is buttermilk. If you’re doing this in a jam jar, just shake it continually until this happens (you may need to take it in shifts with a friend!).