If this turn is not being made at an intersection with a stop sign or left turn lane, I'd be reluctant to do that in traffic downhill or not. I'll do similar things, but I ride or stop on the shoulder until there is a gap in traffic that allows me to cross the lane(s) and make the turn without any concern for traffic.

But as far as signaling, turning and braking, I signal first, then brake and turn with both hands on the bars. If it's so steep you can't really take your hands off the brakes to signal, and so much traffic that you risk getting run over. I'd figure out a different way to handle this. Even using a crosswalk at the next opportunity to get to the other side of the road.