A Ritual Summon (Gishiki Shoukan 儀式召喚) is the act of Special Summoning a Ritual Monster that is conducted through the effect of a Ritual Spell Card. These Summons usually require Tributes greater than or equal to the Level of the monster being Summoned from your side of the field or your hand. These Tributes are treated as a Card Effect rather than a Cost. However, some cards, such as "End of the World" require them to have the exact level requirement. With the effect of "Advanced Ritual Art" you can send Monsters from your Deck to the Graveyard instead of from your hand.
A Monster that was properly Ritual Summoned can be Special Summoned from the Graveyard unless otherwise dictated by the card's text.
How to Summon a Ritual Monster
1. During your Main Phase 1 or 2, when you have a Ritual Monster in your hand and its required Ritual Spell Card in your hand or face-down on the field and have another monster(s) on the field (face-up or face-down) or hand whose total Level is equal to the Level of that Ritual Monster as well as an empty Monster Card Zone you can activate the Ritual Spell Card.
2. Send monsters from your hand or your side of the field to the Graveyard whose total Level is equal to the Level of the Ritual Monster you wish to Ritual Summon. Monsters used in a Ritual Summon are treated as Tributes.
3. After sending the Tributed Monsters to the Graveyard, play the Ritual Monster Card onto the field in either face-up Attack or Defense Position. Finally, place the Ritual Spell Card in the Graveyard.