Practice riding in an empty parking lot where you can get used the feel of another rider, and turning with the extra weight. Try constant speed turns say at 15-20 MPH (or the KPH equivalent), then proceed to slow speed turns, and finally a U-turn.
I would suggest beginning with a set of clear instructions for your rider.
For example, last to get on, and last to get off.
Make sure you have one of the brakes squeezed when someone gets on. If not, the bike could suddenly roll forward or back.
When I dismount with my wife on my scooter I firmly hold the brake, hit the kill switch, turn my wheel all the way to the left, turn the ignition off, and kick out the side stand. At that point it's safe for me to get off without the bike rolling, and equally safe for my wife.
Instruct your rider to hold you about your waist, or if there is a side or rear bar on the seat to hold that. When turning they may lean into the turns with you, but not too much, if at all, but make sure it s clear that they should NEVER lean in the opposite direction that you are leaning or turning in.
I also fine it useful to instruct them not to wiggle or waver when at a stop -- say at a light or busy intersection as it can throw your balance off.
Give yourself more time to come to a stop at a light, stop sign or behind a braking vehicle as your stopping distance will be longer due to the extra weight. Because of this be more mindful of following distances. The two second rule (minimum) is never more important when you have a rider.
If you are counter-steering let them know that you will tend to accelerate into a turn and that that is actually normal.
Last, reassure your rider with this simple rule:
You control the bike, but the passenger controls the rider. Create clear signals for your passenger so that they can let you know if you are riding too fast, they are uncomfortable, or wish for you to pull over and stop.
Hope this helps!
-Wolf
