Then, you figure out how to make it stay still.
An upended stool is a good start, but any air movement whatsoever will make it drift away. So you tether it with a makeshift clip twist tied to the stool. And you get to work.
After hours of effort, you will be halfway done.
And then, just as you feel like you are making good progress, the balloon will shift and suddenly be punctured by the twist tie. So your final product looks like this.
Now, if you'll excuse me, I'm going to weep in a corner for a while.