How do you use the Egyptian god cards?

Note, the God cards can't be used in a real duel, only for fun. Slifer: You must offer 3 Tributes to Tribute Summon this card. The ATK and DEF of this card becomes the number of cards in your Hand x 1000. If this card is Special Summoned, it is sent to the Graveyard during the End Phase. While this card is face-up on the field, it cannot be targeted by Spells, Traps or Monsters that designate a target. Additionally, when your opponent successfully Summons or Flip Summons a Monster decrease the ATK and DEF by 2000, if either attack or defense becomes 0 then the monster is destroyed.

Obelisk: You must offer 3 Tributes to Tribute Summon this card. If this card is Special Summoned, it is sent to the Graveyard during the End Phase. While this card is face-up on the field, it cannot be targeted by Spells, Traps or Monsters that designate a target. By offering 2 Monsters on your Field as Tributes, all Monsters on your opponent's Field are destroyed. Ra: You must offer 3 Tributes to Tribute Summon this card. If this card is Special Summoned, it is sent to the Graveyard during the End Phase. While this card is face-up on the field, it cannot be targeted by Spells, Traps or Monsters that designate a target. You may pay any number of life points to increase the attack of this card by the amount you paid. By paying 1000 life points you may destroy any monster.