Fuse it to your stabilizer.

What I know about this is actually nothing but doesn't the Wonder Under accomplish the same thing as the stabilizer? Just a different product and probably heavier but will offer "stability" of the fabric? I'd be concerned that trying to peel it off will distort the fabric. Count it up as a learning experience and move on.

I would be afraid of the distorting of the fabric too. I would probably just iron on the lightest weight stabilizer I could find. Also, if you have leftover flannel, use it and discard the piece that you messed up on.