Janna

this is really vague

grab because got nerfed to shit

max e

protect ur adc

make sure you have the windspeaker keystone for better shield and buffs on the shield, get the %mana restoration and increased cdr cap masteries in cunning too. rune-wise, personally i take flat ap marks and quints for better imediate stats on abilities(maybe not optimal but i like it for all my squish supports) with armor seals(im most likely swapping those out soon for flat health or something) and mana regen glyphs for more shield spamming

playstyle wise, i play her(and assume the few people that still main her also) play very passively for the most part, your squish AF so stay behind your adc max e and keep shields on them for every trade and to mitigate damage from enemy engage and poke(obviously) your kit is very reliable peel and that will be your biggest attribute in fights keep shields on the biggest ally threat(usually this will be your adc since the ad boost on the shield and the def boost from windspeakers will be VERY helpful for them) and keep any divers away from your carries as much as possible
 for items i prefer frost queens for the pick potential and for alerting you if there an enemy waiting to two hit you in a brush somewhere when you inevitably have to leave your teammates side to get some wards out but talisman is probably a better choice overall

one of your ultimate goals is to basically take next to no damage and get tons of assists which is what makes mejai a viable option on her, although i havent been getting it recently other then that i like the sightstone upgrade for lowering the frost queens cd for more scouting/pick potential, and i typically stick to locket,zekes, and ardent sensor in that order, unless other needs arise, and the game basically never lasts after that

a final note is i usually like using monsoon,aka F*** your engage, as a peel tool and steady mid fight heal as well over using it for engaging purposes or just for a simple heal, but this is obviously purely my personal preference 

hope it helps :D

She's not that hard really. It's all about timing and predicting though.