When you Summon “Guardian Elma”, her effect activates and you choose the Equip Spell Card you want. Your opponent can then chain to the effect and, with an effect like “Disappear”, can remove the Equip Spell Card so that “Guardian Elma”’s effect disappears.
You cannot equip inappropriate Equip Spell Cards like “Premature Burial” or “Snatch Steal” to “Guardian Elma” with her effect.
You can equip a card with a cost, like “Wicked-Breaking Flamberge - Baou” to “Guardian Elma” with her effect, and you don’t have to pay the cost because the Equip Spell Card is not considered “activated”.
“Morale Boost”’s effect is applied when you equip “Guardian Elma” with a Spell Card using her effect.