Ahh I love it! I personally use a small steam cleaner with a hose attachment and an electric pump to do it. Here’s one I did last week. I have a few pics riding it too if anyone wants I also did an Intex Explorer 200 like this but my gf accidentally burst it...
Enjoy!
First pic is from internet but shows how small it actually was
The backrest has at least doubled in size, the seat itself is hard to stretch as the vinyl rips at the start/end of the seams.. but I really like the plump look now