This is my first guide. I've been experimenting with Phinn. I think he has lots of potential in a well-coordinated team. His big downside is his move speed - dang is he SLOW! Anyway, here are some ideas. Feedback welcome!

Start with Ironguard Contract. Get Sprint Boots ASAP. Then add some basic health and defense.

Phinn is slow so Travel Boots are essential - get them next. Then, depending on the enemy team, build towards your first tier 3 defense item. Generally this will be Fountain of Renewal unless you are facing off against an Ardan or Adagio, in which case Crucible is advised.

Your core support items will be Fountain of Renewal and Crucible. If the enemy team is relying on attack speed (e.g., Krul with Breaking Point) then rush Atlas Pauldron. Otherwise, Coat of Plates is probably sufficient. Don't forget to put down lots of Scout Traps!

Late game, build warhorn if you can for the health and speed boost. (Exception: if facing a Taka then rush Contraption mid-game.) If you want to deal more damage, build Broken Myth and grab a Crystal Infusion. Otherwise, grab flares to avoid ambushes and infuse when needed.