Hehe, that was me, actually. It's been about a year since I did it and it still feels great, I can barely tell where the old foam ends and the nerf begins! It feels like the stock seat foam should. The hardest part is trimming it to the right shape of the bolster. I used scissors, but an electric carver might be a little easier to use.
Basically you cut the cancerous foam out of the seat in as much of a clean square shape hole as possible. Go ahead and skin the footballs, then cut a similarly shaped square (I guess it would be more of a cube, really) out of the nerf only a little bigger than the whole you made in the seat (the foam is more forgiving if you have too much of it than if you have too little) You will want to make sure it sticks out when you glue it into the seat, and then trim it down to shape. If you trim it down too far, you can always add a small strip to cover it and trim it down again, but the more you do this, the greater the risk of lumpiness. I did repairs on both seats and I found it to be very helpful to have the other seat for reference while shaping the foam.
Now, for adhesive I used Gorilla glue, but I've heard 3M makes a spray adhesive for truck interiors that supposedly works pretty well. I have also heard that rubber contact cement will work, but I somehow doubt it will last as long.
I probably spent about $20 on 1 jar of adhesive and 3 Nerf balls, and with trial and error, about 3 days rebuilding the foam. Seriously the best 20 bucks I've ever invested in my car!