Seat Foam replacement - how to?

[brittleware]

Hi, I've a new foam on the way for Fiat Ulysse (C8 Pug 807 in disguise) as the old driver's side one is saggy. It's only the seat squab, not the backrest. I'm assuming that the foam is glued (or sprayed) onto the sear cloth. Has anyone any experience of doing this? TIA!