Players need to increase the happiness of Pokemon to make them evolve in Pokemon Sun and Moon, just like the previous games. It could be quite tedious and take a long time the normal way because players need to give the Pokemon a Soothe Bell and walk it. He also needs to use vitamins or Berries and win several battles without fainting. However, a Redditor named Leafeon111 has posted a tip on how to increase happiness and max out friendship gauges quickly.
According to Leafeon111, to increase Pokemon the quick and easy way, the player needs to choose a Pokemon which can evolve with happiness. This includes Pichu, Eevee, and Munchlax among others. The player then needs to find two TM items that that Pokemon can learn. The player needs to use up one move slot of that Pokemon and have it learn a chosen TM.
Next, the player should replace the first learned TM with the other one and rinse and repeat until the Pokemon happiness is raised. Once the Pokemon level is up, it should evolve. It should be noted though that Eevee will become Espeon during daytime and Umbreon at night. Pichu will become Pikachu while Munchlax will become Snorlax. This easy method works because a Pokemon's happiness is raised when it learns a TM move, which does not expire when used. This trick is an efficient way to max out the player's companions' friendship levels.
There are other ways to evolve Eevee as well. The player can evolve Eevee into Leafeon by having it in your party and going to Lush Jungle near Route 8. There is a big rock which will say the surface is covered with moss, it feels soft to the touch. Just keep on fighting the Pokemon in the area until it evolves into Leafeon.
Lastly, to evolve it to Glaceon, you need to follow the same instructions on how to evolve Leafeon, but this time, you will need an icy rock which can be found at the Pokemon League. It should say it is a rock that is covered in ice, touching it seems likely to freeze you solid!