Ritual 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 performed as part of the Ritual Spell Card's effect resolution rather than as a Cost.
Some cards, such as "End of the World", require the total Level of the Tributed monsters to exactly equal the Level of the Ritual Monster. Some Ritual Spell Cards have special requirements to perform the Ritual Summon; "Advanced Ritual Art" sends monsters from the Main Deck to the Graveyard to perform the Ritual Summon; "Gishki Photomirror" does not Tribute any monsters, instead requiring the player to pay 500 Life Points x the Level of the Ritual Monster.
A monster that was Ritual Summoned can be Special Summoned from the Graveyard, unless otherwise stated.
Performing a Ritual Summon
During an open game state in the Main Phase, if the turn player has a Ritual Spell Card in their hand or Set on the field, an appropriate Ritual Monster in their hand, and monsters in their hand or on their side of the field whose total Levels meet the requirements of the Ritual Spell Card, that player can activate the Ritual Spell Card.
Upon resolution of the Ritual Spell Card, the player that activated the Ritual Spell Card Tributes monsters from a location(s) specified on the Ritual Spell Card, whose total Levels meet the requirements of the Ritual Spell Card. That player Special Summons the Ritual Monster to their side of the field, face-up.
Tributing the monsters and Ritual Summoning the Ritual Monster occur simultaneously. Additionally, if either the monsters cannot be Tributed or the monster cannot be Ritual Summoned, neither is performed.