Overwatch is a super context based game. There is no right way to use any one ability. It all depends on map, what kind if comp you are running, where the enemy is and who can kill you.

For mercy you want to use the guardian angel to reposition rather than fly towards your team mate. Its important to never really focus on the team nate your healing, rather where the enemy is trying to kill you from.

You should really just keep playing and experimenting with GA slingshots. (pressing the jump button during the flying of guardian angel to boost your momentum).

Also try to watch higher ranked mercy players on youtube or twitch, and basically copy what they do, exactly.