Help me not ruin my $20,000k sectional before my husband gets home

First time dyeing anything ever. We have a very large Restoration Hardware white linen sectional. Between kids and my dog, it's time to dye it black. I was able to take off all of the covers. After watching some videos, I realized it would stain the bathtub, so that's out. Using a kiddie pool outside won't get me hot water. I really don't know what I am doing. Any advice would be greatly appreciated. 🙏🏻

• edit. I washed the cushions. I used FOLEX - some spot remover the husband say on Instagram. It actually worked! I wanted to share because I can't believe that it worked. Far from perfect, but still fine for the game room area. Photos in comments below. ⬇️