One of the advantages of knowing how to make butter is that when you take your eye off the whipped cream for too long and ruin it, you don't have to pour it down the sink.
To make butter deliberately (or by accident), beat cream in a food processor until it whips, then keep beating until it collapses again. If you stop the beaters at this point you'll see a curdled-looking mess of yellow buttery lumps and white buttermilk. Keep beating until the yellow lumps come together. It'll probably be soft and the beaters will smear it out, rather than a hard lump that goes "thud thud", but stop anyway.
Pour or scrape it all out into a sieve over a dish. The thin buttermilk will pour into the dish - keep it to make scones or soda bread. The butter will stay in the sieve, but it still contains buttermilk in little holes like a sponge. Squeeze the butter in your clean hands to get the buttermilk out, and rinse it in clean water. Keep squeezing and rinsing it - if you leave the buttermilk behind you butter will go rancid.
Once you have pure butter, put it in a dish and use it. It's delicious - cleaner and fresher-tasting than shop-bought butter. And so much better than pouring your ruined whipped cream down the sink.