Eye level, arm's length
A phone below your chin looks up your nose; above your eyes looks down on you. Eye level reads as a conversation.
Look at the lens
Your eyes want to watch yourself on screen. Put a small sticker next to the lens and talk to it.
Talk to one person
Picture a friend who'd care about what you're saying. You'll speak more naturally than if you address an audience.
Bullet points, not a script
Unless you use a teleprompter, memorised scripts sound memorised. Know your three points and talk.
Three takes
The first is stiff, the second is warm, the third is usually best. Keep them all and choose later.