Plan what you want to say, relax, walk up to them and start. I've managed a way of just saying what I need to without thinking. Best to have numerous things in case a subject comes up. Work on the ground and improvise the filling, roll with it, express views.
Concentrate on walking and taking note of anything that can get in your way, if he should look your way, give a smile. When your brain goes on strike, breathe, and say what you want on your next breath to exit.
Breathe in deeply, breathe out saying the most basic of all communication "hi"
I know - it's never easy but it's a start