I did the heavy armor side of the smithing tree, and I want to smith a set of forsworn armor to legendary, but only if I don't have to waste a bunch of perks. What perk/perks do I need to smith it to legendary? My smith skill is 100.

Forsworn is leather armor so you can already make it legendary, I think. If not then it's the light armor side advance armor.

I could only get it up to flawless. Must be advanced armor then. Too many perks to bother with. I only wanted it to help Aela from dying so much.

Well Aela wears Heavy Armor by default anyways; she's wearing Ancient Nordic Armor, which has the same stats as Iron. Give her Steel+ armor?

Leather armors like Forsworn, as well as Iron, I think, only go to flawless. I've filled the perk tree. Sadly, 'tis impossible to have legendary Forsworn armor (unless there's something about potions that I don't know).

196 Smithing is your goal for Legendary quality items, of any kind.

Ah, thanks; I'm going to have to train my alchemy then.