If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If an instant or sorcery card you cast this way goes to a zone other than exile or a graveyard, perhaps because one of its abilities says to put it into its owner’s hand, it won’t be exiled. This is true even if that same card would be put into a graveyard later in the turn.

If an instant or sorcery card you cast this way is countered, it will be exiled.

If you cast a spell “without paying its mana cost”, you can’t choose to cast it for any alternative costs. You can, however, pay additional costs. If the spell has any mandatory additional costs, those must be paid to cast the spell.

You cast the spell while Daring Waverider’s ability is resolving and still on the stack. You can’t wait to cast it later in the turn.