Hi Cate – don’t worry! This is a very minor mistake and easy to fix.
There are multiple ways you could fix it, but if only the first side of the baffle is attached to the seat, the easiest thing to do is to cut off half of the baffle, and then cut a rectangle of scrap fabric 2-3 centimeters (~1 inch) wider than the part you cut off, and then weld the new piece to the part of the baffle that is already attached to the seat fabric. That will effectively flip the TPU from one side to the other and allow you to carry on following the instructions without having to do anything else differently.
If you’re not sure what I mean here, let me know and I’ll post a picture. Cheers 🙂