How to stop a new-style toilet from running

My toilet has been running for a while. Not all the time but now and again. This means (usually) that there’s crud somewhere under the flapper, so I gave it a good wipe down (lots of black mold on that rubber gasket, it’s black for a reason) and it stopped. But I wasn’t sure how to fix a permanent running water problem. Fortunately this guy does. It’s a great step-by-step for you.