You must follow the normal timing permissions and restrictions of the card you cast from your graveyard.

You must pay the costs to cast that card. If it has an alternative cost, you may cast it for that cost instead.

Once you begin to cast the card, losing control of Kess won't affect the spell. You can finish casting it as normal and it will be exiled if it would be put into your graveyard.

If you cast a card from your graveyard using another permission (such as that of flashback), Kess's effect doesn't apply. You can cast another instant or sorcery card from your graveyard.

If you cast one instant or sorcery card from your graveyard and then have a new Kess come under your control in the same turn, you may cast another instant or sorcery card from your graveyard that turn.

If a sorcery card is put into your graveyard during your main phase and the stack is empty, you have a chance to cast it before any player may attempt to remove that card from your graveyard.

If a spell cast this way is put into another zone after it resolves (perhaps because it had buyback), it's considered a new object. If that card later moves to your graveyard, it won't be exiled.