Source: https://www.youtube.com/watch?v=rafB8p9Eddo

Spell Casters lose their spell slots, they can cast their spells at will. Instead when they cast a spell, they make an Ability Check using their Spell Casting Modifier. DC = 10 + Spell Level.

If they pass the check they cast the spell as normal. If they fail, the spell is not cast and you “forget” the spell, meaning you are unable to cast the spell until your next long rest.

You can also do Critical Successes and Failures on the Spell. (Will Get consequences Later)

Lastly, if they fail their Spell Check, the caster can spend hit points equal to the difference they would need to make the Check a success.