Everyone knows /summon command without it you cant summon custom mobs. So lets start with something simple:
/summon Skeleton {Riding:{id:Slime}}
This means that we spawn a skeleton riding a slime. ( {} Is a data tag)
But this:
/summon Skeleton ~ ~2 ~ {Equipment:[{id:286},{},{},{},{id:314}], Riding:{id:Zombie}}
Means that we spawned a Skeleton wearing golden helmet and a golden axe.Equipment means that mob is wearing that and that. Equipment:[{hand}, {boots}, {legs}, {body}, {helmet}].
Next:
/summon Slime ~ ~2 ~ {Riding:{id:Zombie, Equipment:[{id:257}, {id:301}, {id:300}, {id:299}, {id:298} ]}}
Sot its a slime riding a Zombie but as you see editing the thing you mob rides on it Does not need {} when writing Equipment,Riding etc.
So i explained how typically /summon {} system works so make you own ideas. Vanilla minecraft has alot of things.