You select the card you are activating in your opponent's Graveyard when you activate "Double Spell". "Double Spell" can then be chained to after you announce your selection.

The selected card itself CANNOT be chained to as you are in resolution of the chain at that point.

If you activate "Double Spell" and select a Continuous, Field, or Equip Spell Card, or "Swords of Revealing Light", you place the selected Spell Card in your Spell & Trap Card Zone, so you need 2 vacant Spell & Trap Card Zone spots (1 for "Double Spell", 1 for the selected card). The Continuous/Field/Equip/"Swords of Revealing Light" Spell Card then remains on the field as normal.

The player who activates "Double Spell" pays any costs for the selected Spell Card (for "Delinquent Duo", "Tribute to the Doomed", etc.).

You cannot select a Spell Card with "Double Spell" that cannot be activated, such as an Equip Spell Card when no monsters are on the field.

You cannot use "Double Spell" while "Necrovalley" is in effect.The latest print of "Necrovalley" has Problem-Solving Card Text which states that it negates the effects of cards that move cards in the Graveyard to other locations, so the effect can be activated but will be negated.

Curse of Darkness:" When "Curse of Darkness" is active, and you activate "Double Spell", you take 1000 damage for activating "Double Spell" but not for the target of "Double Spell", because it is not considered "activated".

Spell Absorption:" A Spell Card used from the Graveyard by the effect of "Double Spell" is not considered "activated", so you will only gain Life Points for "Spell Absorption" for "Double Spell" itself, not for the card used from the Graveyard.

Spell Counter: When "Double Spell" is played, you get a Spell Counter on this monster for the activation of "Double Spell", but not for the target of "Double Spell" because the target is never activated; so you get 1 Spell Counter on this monster.

If the selected Spell Card is Ritual, Normal, or Quick-Play (except for "Swords of Revealing Light"), it remains in your opponent's Graveyard and is not placed on the field, so you only need 1 vacant Spell & Trap Card Zone spot (for "Double Spell" itself) to activate "Double Spell".The latest print of "Double Spell" has Problem-Solving Card Text that specifies that the target Spell Card will be placed on the field regardless of card type. As such, a vacant Zone other than that of "Double Spell" is required to activate this card.