Before evolving
All these evolutions also take the species' usual Candy. The Pokémon's screen tells what's missing: an item, kilometers, a number of catches. Once the condition is met, the Evolve button lights up.
Evolution items
Items are mostly rewards: spinning PokéStops (rarely), Field Research breakthroughs, Team GO Rocket and its leaders, Trainer Battles and event research. A Sinnoh Stone works for 18 evolutions: save it for the one you need most.
The buddy
For a buddy evolution, pick the Pokémon as your buddy and do what's asked: walk, catch Pokémon of a type, win raids, earn hearts (playing, feeding, taking snapshots). The counter only starts once the Pokémon is your buddy.
Lure Modules
For Glaceon, Leafeon, Magnezone, Probopass, Vikavolt, Crabominable or Goodra, put a Glacial, Mossy, Magnetic or Rainy Lure Module on a PokéStop, or use another player's, and evolve in range.
Trades
Trading a Kadabra, Machoke, Graveler, Haunter, Boldore or Gurdurr (and a few others) lets them evolve for no Candy. Trade the first stage: the perk still applies to the last evolution.
Frequently asked questions
Does the Eevee name trick still work?
Yes, but only once per evolution and per account: rename Eevee (Rainer, Sparky, Pyro, Sakura, Tamao, Linnea, Rea or Kira), close and reopen its screen, and check the silhouette changed before evolving.
Where do you get a Sinnoh Stone or an Unova Stone?
Mostly as rewards: battles against Team GO Rocket and its leaders, Field Research breakthroughs, Trainer Battles and some event research.
How do you evolve “at night”?
The game follows your phone's clock. When the map switches to night mode, with a dark background, a “night” evolution works; in day mode, a “day” one.