There's not much to that stuff. Lay it down. Done. No glue or anything. I spent more time using Peel N Seal from my local home-improvement store to supplement that underlayment, because yours will only cover a portion of the floor. Glad I did. In hindsight I would have just used 100% Peel N Seal and skipped that stuff you bought, but mine was non-returnable so I went ahead and used it up.

These cars generate a LOT of heat from the floor, with the trans and exhaust running so close and almost zero heat shielding underneath. You definitely want that deadening in there at a bare minimum, plus more. For sound it's a relatively pointless exercise because it's a convertible...plus many of our cars run dual exhaust and louder mufflers. So it's not like you're building a quiet Cadillac...but focus on heat isolation, and from that perspective it is VERY nice to have 100% of the tunnel & firewall covered below the carpet...and that underlayment kit you purchased ignores the majority of the tunnel & much of the firewall.