Leonardo's Technique

If a creature spell's sneak cost was paid, the creature it becomes enters tapped and attacking the same player, planeswalker, or battle as the creature that was returned to its owner's hand to pay its sneak cost. This is a rule specific to sneak; in other cases, when a creature is put onto the battlefield attacking, that creature's controller chooses which player, planeswalker, or battle it's attacking.

If a card in a graveyard has X generic mana in its mana cost, X is 0 for the purpose of determining its mana value.

Spells can only be cast for their sneak costs any time you could play an instant during the declare blockers step on your turn (after your opponent has decided whether to block).