Search your library for a permanent card with mana value X or less, put it onto the battlefield, then shuffle.
A permanent card is an artifact, creature, enchantment, land, or planeswalker card.
You may always find a permanent card with converted mana cost 0. (If X is 0, that’s all you can find.) Cards with no mana cost (such as land cards), cards with mana cost {0}, and cards with mana cost {X} all have converted mana cost 0.