Hi there im currently the "best" morgana player acording to league graph:Take it with a grain of salt ofc but hopefully this gives abit credit to what i say :)
Ive played morg mid to around GM last season and right now im master with a decent win rate.

Your pre 5 is the weakest state. Ironically lv 1 is actualy quite safe cause you outpush most champions. Your main goal is to push and safely poke while ur waves are pushing. Do not take heavy trades early cause dmg output is low and you DO NOT WANT TO USE MANA FOR Q IF NOT ABSOLUTELY NECESSARILY. Try to outlast with ur mana against mage opponents so u can push the last wave in and get a free back.
If you can back with a lost chapter then great, if not 2 amblifine tomb is fine to. After lv 6 most champion that falls below 80% hp you can kill with "1" rotation. The thing is that with max lv w after hitting a q, with w on top and u press r. Once the stun pops off ur q is off cd again so its actualy "2" rotations. If they dont have f they most likely die cause ur spellshield will prevent the disengage cc from most mages at this point.
From lv 7-11 point of the laning phase you should dominate if not going even against almost any champ if u are not behind after first back.

Here after laning phase your goul is to have sorc boots, ludens and hopefully oblivion orb. THis is your first and one of the biggest powerspike. At this point nobody is tanky enough and any q w you hit is atleast 30-50% chunk. Here you are looking to group and skirmish 2v2, 3v3 or 4v4. Look toward drake/herald or first/secund towers. The good news dont end here. Your power dont go down from here but up. Morg is one of the best mid game powerspiker in the game and at 3 item you are by far one of the best champion. If you are ahead look to complete rabba after morello( if they have heal, otherwise leave it at orb), if not you can consider void( if they start building mr), liandries or rabba. I find it in order to keep ur overwhelming presence longer going rabba over liandries is beter.

Unforunatly while you will one shot none tanks with qw in the late game, morg still falls off compared to other mages. The main problem is that while in mid game you dominate adc cause they dont have the dmg to burst you down. Here they got the dps + life steal which means if you miss q the punishment is immense, also they will most likely have qss which basicly nulify ur first q. And in late game where there is so much dmg and fights go on so fast you often dont get the time to do many rotations. Fret not while ur scale off abit you are still good to go. Full build with liandries still shred both tank and adc alike. But you have to MIND YOUR POSITION. Here amplifies the weakness of zhonya on morg. Your r is basicly only a self peel at this point, dont even think about the stun part thats just to greedy. Any champ close enough to dps you will shred you. Staying stasis is basicly a death sentence. You need to always be on the move, pink is important cause it create blind spot for you to q from. MOvement is key and u become more of a utility dmg dealer rather then the dominant 1v3 kind of dmg dealer from mid game.