How do you contract and relax the sphincter muscle?

I used to think that you do the same thing that you do to stop yourself from peeing which is what you do when doing kegel exercises.

But when looking into it it seems that this is wrong? I'm always hearing that to contract and relax your sphincter you have to do the same thing you do to stop crap or a fart from coming out, I never hear it being said that you do the same thing when you stop peeing.

I'm sure that whenever I stop crap or a fart from coming out that I do the same exact thing to stop myself peeing.

So I thought you must have to use different muscles then, like maybe clenching your buttoks together, but this raises my ass up from the chair, and I've read that your ass should not do that.

How on Earth do I contract and relax the sphincter muscle?