I have helped a couple of adults who had never been on a bike in their lives.
As above, I would remove the pedals and have them ride it like a hobby horse until they get the idea.
The one thing that I have found to be the biggest challenge is to get them to look where they want to go. For some reason, after they go a few feet, they always want to look down at the front wheel. Where you look is where you will go, so look ahead, not down.