SP, also known as Skill Points, is a key mechanic in Fire Emblem Engage. It's how your characters can inherit abilities from the Emblem Rings they've bonded with, unlocking powerful passives they can use in battle. But finding SP can be difficult in Fire Emblem Engage.
In Fire Emblem Engage, the best way to farm SP is to farm SP.
The best way to ensure that your characters earn SP regularly is to give them a Bond Ring. This prevents them from purchasing any passive abilities they can earn when bonding with one of the more powerful Emblem Rings.
Make sure every character you want to have to acquire SP is wearing a Bond Ring before taking them on a quest. These battles can occur in Skirmishes, which are often the best way to earn experience, gold, and SP before progressing further in the game.
The Skirmishes will continue to appear each time you defeat one, so you should always have a challenge ahead of you on the map. You can always switch your tactics if you have a private objective for your characters to reach before moving on.
Another great way to get involved in these battles is to have a character wear the Emblem Ring for Micaiah, the Emblem of Dawn. Her special ability, Great Sacrifice, allows the wielder to heal all other enemies in combat. As long as multiple characters are severely damaged, the character wearing this ring can gain a lot of experience and SP in a single turn.
Related: The Best Fire Emblem Engage Classes for Each Character
Farming SP can take a fair amount of time, but focusing on Skirmishes is likely your best bet. If you don't like the Bond Rings you have, feel free to visit Somniel to acquire more using any spare Bond Fragments you have.