How to Summon a Tamed Modded Entity

So basically I'm making a pet mod

I have a GUI that has a slot for your pet item and two buttons, spawn and despawn pet

I need help with spawning the entity and making it follow the player

I can spawn it currently, but it won't follow the player and after you do it once, the entity disappears and you can't resummon or despawn it.

How do I summon the entity, make it follow you, only be able to summon it once, and be able to despawn it?