Question About Kegels

I want to start Kegels because I'm having a hard time getting an erection firm enough to have sex. It's only getting worse as I get older.

Isolating the muscle is challenging though. I've read that when it's contracting your anus should also contract. But I also read that the buttocks shouldn't contract. So where exactly is the line drawn? I know that so far I can't get my penis to budge without tensing some other muscle.

I've done web searches but everyone just reiterates the same stuff. I'd appreciate some pointers. Thanks.