How do I make an enchant?

I'm trying to make some enchantments, and possibly modify an existing one, but I have no clue what to do.

1. I'm trying to make an enchantment that makes an entity wearing it have speed 1.
2. I'm trying to make a bane of enchant for illagers
3. I'm trying to make bane of arthropods apply to more mobs

Unfortunately, I have no clue how to do this, and ya'll are genius', so hopefully you know what to do.