How to manually focus with a Nikon D90 whilst filming?

So whenever I film with my Nikon D90 I find that if people come closer than what they started at they become out of focus. I assume the only way to refocus is manual so can someone talk through how this is done?

Flip the switch from AF to MF, turn the focus ring on the lens as needed.

On most lenses, there's a physical switch to change the lens from autofocus to manual focus. Put yours in the "MF" position.
Then turn the focus ring on the lens.

It helps to practice ahead of time, so you know which way to turn the focus ring if the subject gets closer or further away.

I agree