Keep the Pokémon in your party
Walk around with the Pokémon in your party
Level up the Pokémon
Use the Pokémon in battles
Avoid letting the Pokémon faint
Feed the Pokémon items that raise friendship
Use a Soothe Bell if available
Heal the Pokémon regularly
Catch it in a Friend Ball if possible
Avoid using bitter healing items on it
Let it hold a friendship-boosting item if supported by your version/modpack