Step 1: Materials
Use anything soft and fairly inert for the padding. I personally enjoy using my "softballs", as they're readily available and upcycled.
Make sure you check the following instructable to learn how to make them: https://m.instructables.com/id/Softballs-Hack-Your-Garbage/?ALLSTEPS
Use any soft and strong material for the outer case. Prefer something fairly natural over synthetic stuff: you don't want to turn into a ball of sweat... Also, try to reuse stuff lying around.
I had an old pierced airbed I couldn't get to fix, perfect!
Step 2: Prepare the Case (here, the Airbed)
Cut one side edge open, preferably the shortest one.
If your airbed has inner strips of material that connects its sides together, cut them off to turn the bed into a giant sack.
Step 3: Filling Up
The correct amount is to everyone's discretion but I'd recommend quite a bit if you want to be able to shape it easily. Also, don't forget that it compresses quite a lot when you sit on it.
Once you're satisfied with the shape and size, close it up.
Use any binding technique you like or, as I lazily did, simply roll the top over itself.
Step 4: Sorted.
Enjoy a nice seat, free, upcycled, perfect.
Thank you and happy DIY!