For more detailed instructions, go to the above link. However, essentially you just add yogurt to whipping cream, stir, let it hang out overnight unrefridgerated to thicken, then mix (we used a stand mixer) until the buttermilk and butter separate. We probably mixed it for about 5 minutes or so at a pretty high speed before it started to turn just a little yellow and separate--have faith, it will get there.
Once your butter looks like the picture above, you have to wash it to get the buttermilk out. We went through several rinses (you add ice water to the mix, slosh it around, and then drain it off), and towards the end I finally just picked it all up with my hands and started squeezing--which had the added benefit of making my hands all soft and smooth.
Was it worth the trouble? Eh, there was certainly something about it. I can't say it tasted leaps and bounds better than store-bought butter, but it had a nice tang and it was very creamy.