Step 1: Fill bag with leaves
The leaves need to be jam packed to avoid the bag deflating when they crumble.
Another option that's totally eco would be a coffee bean sack or fleece. Throw it in the garden when you're done and it will surely be gone (or at least grow stuff through the bag) in the next few decades.
Simply get a big piece, join it into a sack shape and tie it with twine.
Instant cushy chair!