Obtain Necrozma
Obtain Solgaleo in Pokémon Ultra Sun or Lunala in Pokémon Ultra Moon
Progress to the point where the Necrozma fusion is available
Use the N-Solarizer to fuse Necrozma with Solgaleo or Lunala
Fuse Necrozma with Solgaleo to get Dusk Mane Necrozma or with Lunala to get Dawn Wings Necrozma
Learn Photon Geyser if needed
Enter battle with fused Necrozma
Use Ultra Burst during battle to transform into Ultra Necrozma
Keep Necrozma fused to maintain access to Ultra Burst