Step 3: what about the beans!
you'll want to fill the beanbag at least 80% full of beans. a normal beanbag chair is not this full, but the sofa will not hold its shape unless it is nearly full. for the earthworm type used as a backrest, you may want to pack it 100%. calculate the volume of the sofa (a cylinder): pi * radius * radius * length. so the garden slug is 56 cubic feet (3.14 * 1.5 * 1.5 * 8). beanbag beans are sold by the cubic foot, so you'll need about 50 cubic feet of them.
where to get the beans! you will need a lot of beans, so the best thing is to find a local styrofoam (EPS) products manufacturer. try the website epsmolders.org to find a list of EPS manufacturers around the country. getting the beans will be a fun excursion unto itself, as you can see below. the technical term for what you want is "expanded polystyrene beads" or "EPS beads" for short. usually they will have both new beans ("virgin beads"), and used beans ("regrind"). virgin costs about 10x as much as regrind, usually more than you'll want to spend on some lunatic project you read about on Instructables. my local foam mfr (Bay Foam in Hayward CA) sells 35 cubic foot bags of regrind for $10 each. they had a huge mountain of them behind their factory! the fancy places online that sell beanbags will tell you that virgin beads have a better feel in the beanbag, but my sofa seems perfectly nice with the regrind. remember: regrind = recycling!
how to find a styrofoam products maker! they do not seem to like to list themselves under "foam" in the phone book, that seems to be reserved for the lowly urethane-foam folks only! instead you'll want to look under "packaging & shipping materials" or whatever else they might be making out of styrofoam, such as architectural trim.
no airbag needed! on your drive back from the foam factory you'll be about as safe as you've ever been in your motor vehicle. now is the time to enter a demolition derby! it took some effort, but i was able to wedge about 120 cubic feet of beans into my minivan after i took most of the seats out.