Floating Mana?

Ok, so my opponent is resolving Blood Moon, I have Abrupt Decay available in hand and Mana to cast it. How do I float GB, let Blood Moon resolve and then cast Abrupt Decay targeting it?
This was an issue that happened earlier, thankfully my opponent was nice enough to concede since I drew the kill on my turn
This was an issue that happened earlier, thankfully my opponent was nice enough to concede since I drew the kill on my turn