I pretty much always start tear with this build for the mana sustain and eventual Fimbulwinter. Its really solid for the slower paced lane we are going for anyways, and when it does get upgraded it will help your build start progressing into its aggressive mid game.

Doran's shield is a good option into a Vayne or Quinn top, or anyone with good AA-based poke. However if you want to sustain better from bad trades and ability spam I think Corrupting pot is a good investment especially since you'll be running TP.

The overall idea of this build is early sustain in lane allowing you to farm/scale up without feeding the enemy laner. By mid game it will scale into a high movement speed Ornn that will control the map and initiate strong team fights. I recommend buying many control wards and grabbing an early sweeper since with this build you will have the ability to dominate vision in both your own jungle and theirs.

There are a few different ways you can do this build, but the most straightforward is Mythic -> Boots -> Dead Man's -> Fimbulwinter -> Bramble vest -> Winter's Approach -> Thornmail. It should be very doable to stick with this build path in most games, only adjust if the enemy comp is special or if your other lanes are struggling and you need to itemize for the enemy team quikly.

There are some different itterations depending on what you need:

[Strong Enemy Healing] Mythic -> Boots -> Bramble -> Fimbulwinter -> Dead Man's -> Thornmail -> Winters Approach

[Top lane camp/Extra lane pressure] Dead Man's Plate (If laner is AP instead of AD go Winters Approach) -> Boots -> Mythic -> Fimbulwinter -> Thornmail -> Warmogs/Gargoyle-Stoneplate/Randuins depending on enemy team

[Heavy DPS Mitigation] Mythic -> Boots -> Fimbulwinter -> Dead Man's or Winters Approach -> Gargoyle Stoneplate -> Thornmail or Warmogs

[Ornn Heavy Magic DPS] Mythic -> Boots -> Demonic Embrace -> Abyssal Mask -> Thornmail -> Warmogs (can build before Thornmail if you don't need greivous as badly)

Boot Alternatives:
Generally you will want Mercury Treads almost always unless you are against an AA-based laner, then go for Steelcaps. However, if the enemy team isn't heavily leaning to one side (AP or AD) or you feel you don't need the extra protection, then Boots of Licidity are a fantastic choice. You get a reduced TP and Flash cooldown from the Summoner Spell CD reduction, and you will be able to use ult more frequently. The last option is boots of switfness if you are against primarily slow-based champions or if you just want the ability to get around the map faster. If you are running the situational Phase Rush runes you will likely want either the Swiftness boots or the Lucidity boots.