Mechanically, LB is about on par with Ahri in terms of difficulty, erring slightly on the more difficult side. They follow a similar play patter as well, except LB needs to wait 1.5 seconds after landing her E to unload her burst.
The hardest thing you'll have to get used to with new LB is her ult. Think of it like Karma's ult in that it empowers LB's next ability. You need to R+Q/W/E/R in order to get the effect. LB can also be difficult to last hit and clear waves with. You'll often find yourself under tower in the early game, but once you have the mana to spam W + Q on the wave, it shouldn't be a problem.
LB's combos can vary depending on the situation. Usually a standard burst combo is Q (wait) > W > E, but sometimes leading with E is necessary against mobile targets so that you can W to keep up with them after they dash/blink. This is where having R comes in handy, as it lets you still have a second dash to proc your passive mark and/or escape (though keep in mind that an empowered Q will do more damage to a single target).
Most importantly, be patient when playing LB. She can win almost every trade at level 1 if you start (and land) your E because it procs your passive by itself, leading to an easy Thunderlords + Fresh Blood auto attack proc. This kind of level 1 burst really catches mid laners off guard, but don't be afraid to start W if you need to push the wave early for a level 2 advantage (think matchups like Cass, Viktor, Zilean, etc. where they get an immediate level 2 powerspike).