It's kind of hard to say what you do because you kind of have to play differently depending on the champion you're playing. Most lbs will hit you with q and then jump to you. If you're playing a champion with CC this is the best time for you to engage them. Lb no longer has silence so when timed right you can CC her before she is able to jump back and get a good trade. That being said in lane you know she is going to most likely jump back. So on champions that do not have CC you can prepare to engage onto her as soon as she jumps back to her mark.
It is difficult to say for sure this is how you play against her or this is how you beat her because you kind of have to play each matchup differently. With Ahri I try to charm her when she jumps in or I throw out my q so when she does jump back she gets hit by the true damage. On zed I poke her down and when I am 6 I wait for her to use her w then ult her and kill her. On ezreal I stand further back, poke her down with my qs, and when she engages she is already low so I can counter engage and win the trade.
One essential thing for each match up (at least in my opinion) is to get boots. If a lb Qs you she is most likely going to try to jump onto you right after so if you see the q coming you can immediately start backing up so when she jumps at you, you are already out of range and she just wastes mana.
Also try to fight when her w is down. She relies on it to do a lot of damage and as an escape so when it is down she can't get away from you and she loses a lot of damage.