Get The Screenwriter’s Bible, it's the go-to for questions like this.
I've only studied screenwriting (I'm a novelist) and I've never sold one. But I would argue that you are trying to direct the scene, which is a no-no with screenwriting. Directors don't like to be told how to shoot their scenes.
Unless you have some absolutely crucial reason for why their lips must not be visible in this scene, I would write it like:
A grassy field extends to the horizon. In the distance, Bill and Ben ride quad bikes.
Dude, this is awesome!
I'm pooping my pants, man, quad bikes scare the crap out of me.
It's up to the director (and dictated by a number of factors like, the terrain of the location, the camera equipment available, etc. etc.) to decide how your scene is shot and whether to film them at a distance and use voice-overs or pan in for a close up.
But if you have good reason for directing how your scene should be shot, it may be worth editing your question to include that information.