Catch or obtain an Eevee
Set Eevee as your buddy and walk 10 km if you want to use the name trick only once
Rename Eevee to Sparky before evolving it for a guaranteed Jolteon, if the name trick is still available to you
Evolve Eevee normally and get Jolteon randomly from the Eevee evolution pool
Use 25 Eevee Candy to evolve Eevee into Jolteon
Check the evolution preview to confirm Jolteon is available before evolving
Use a Mossy Lure, Glacial Lure, or other special lure only if you are targeting a different Eeveelution, not Jolteon
Catch more Eevee during events to stock up on Candy and increase your chances
Use Pinap Berries when catching Eevee to earn extra Candy
Trade for Eevee if you do not have one to evolve