In response to a spell being cast on you that you would rather not be in effect, you proclaim "no u." You cancel whatever 4th level (or lower) spell they had cast at or on you and send it right back at them.
When sending the spell back at your enemy, treat it as if you had cast the spell yourself at 4th level. You do not need to be able to cast it yourself.
If attempting to deflect a spell of 5th level or higher, roll a spellcasting check against the casters DC.   Ascended Spell. This spell costs 2 Spell slots to cast. A 3rd-level spell slot and a 2nd-level spell slot.