My Rotation is:

Y - (Dragonflight) and Q - (Drop in with your Destroyer Form)
W - (Burn enemys)
E - (Slow and weaken enemys)

escape from fight and let your team do the rest. if needed support them with a molten flame (Q) from the background or use R and fly over the teamfight

switch to a free lane, defend and wait until your team starts a new fight (if its a long fight repeat all steps - you can also cut the running paths with your R)

here some statistics of the lastest replays (quick match with random players):

LVL 1: use "Incinerate" to stay alive, if you get focused after the drop
LVL 4: your "Incinerate" gets much stronger and now does nice AoE damage
LVL 7: after you drop into Teamfights - "Firestorm" reduces Dragonflight's CD so you can do more drops
LVL 10: do what you want. i took "Burn Beneath my Shadow" and fly over teamfights or cut running paths, but you can chose "Bellowing Roar" if you want. Depends on your playstyle. doesn't matter in this build.
LVL 13: If the enemys try to escape hit them with "Onslaught" to slow them and prevent escaping
LVL 16: do what you did all the time. Drop into teamfights to reduce the enemy damage significant and slow them a long time.
LVL 20: now you have all you need to destroy your enemys even more and win every teamfight!