Having refitted 2 of my own carpets from raw material and seen a pre-packaged set as you described fitted into a friend's 3500 S, I would say this:
1. Remove the carpet from the backing if at all possible and install the underlay first on all but the two mats that go the length of the floor.
2. Then, spray the two surfaces with a spray adhesive that doesn't dry like contact cement
3. You need the flexibility to stretch some of the carpet and prehaps create slits or narrow pie-shaped cutout to make all the curves needed
With the carpet attached to the underlay already you have very little flexibility on the compound curves.