I wouldn't necessarily sit back, being roughly in line with your adc is probably a good idea. This lets you add your AA poke to tradea (which granted isn't much), but since your w passive means you almost certainly move slightly faster than the adc, you can be very slightly in front of them when they're stepping forward to poke so they benefit from your passive, and also very slightly behind them as they kite back at the same time as you, so you can get full use out of your passive.
Also regarding shielding, shield as they auto enemy champs, or when the enemy is about to hit the adc. Just mindlessly spamming the shield can result in a lot of wasted shield and/or tip off the enemy that your adc is about to trade with them.
Flash Q and Flash ult are things that can work if you know you can safely do them with a payoff, but often not worth the risk.
Prepping a Q in advance so it's charged and aiming for where the enemy is going to be trying to escape (rather than using q to initiate) can be an offensive use of Q that's probably more useful than a quick snap cast directly at them. Still, if you're playing janna, theres better picks if you want to be aggressively making plays, her thing is more preventing enemy plays.
