The first thing I do when I see a raw, green rookie is look him square in the eye, make sure I've got his attention, and think to myself, "One day, when I retire, this might well be the guy who buys my business."
The best advice I can give is don't skimp on questions. Both his and yours.
I vividly remember the time I had a cubby show up on the job site. I set him to work digging a trench between two buildings (about 40 feet, so no trencher). I told him to make it four foot [the specs required it], gave him a shovel and walked away.
To my surprise, in far less time I would have ever thought possible, he came to me and said he was finished. I went out to inspect his work.
He said, "You didn't tell me whether it needed to be four feet deep or four feet wide, so I made it both!"
His name is Barry, and ever since I have called him Barry the Backhoe.