I recently replaced the gaskets on my drysuit -- you're right, the neck gasket is the tricky one.
Kokatat's got some good instructions to download from their website....

http://www.kokatat.com/instruction2g1.htm#NECK

: Hi All-

: I’ve got an nice old Kokotat goretex dry suit, but its gaskets are all tired
: & worn.
: I just bought a set of new gaskets to replace them .
: I have done arm & leg gaskets on it before, and a neck gasket on a dry
: top.

: But the neck gasket on the Kokotat seems a lot more difficult to put on.
: Anyone have any tips on how it is done? Do I need any special tools or
: clamps?
: I got a big tube of aquaseal...

: Could I do it in sections?
: Just gluing what I can align & clamp overnight?

: Thanks!

: Jack Gilman