I have done 1(78 camero) and getting ready to begin the build of one for my 89 pathy.
Ill tell you how I went about it for the camero, it's the same method im using for my pathy.
First i removed the old dash, and marked/documented all the wire connections, indluding what voltages, devices, etc(for future/replacement gauges, or incoders.)
this can be very time consuming and some knowlege of electronics will go a long way.
2-Once removed, I marked all the the mounting points, and created a wooden jig that would mount to the firewall. Durring this time i also took critical dimensions on a 2d board, things such as stearing columb, heater/ac components ets.
I cant stress enough how important this stage is, get measurements of evrything.
3-once i had all relative info/dimmensions I began fabricating. Things you need to know ahead of time, types of replacement gauges, type of stereo, heater vents/hosing runs, switch placements etc. My background is in electrical engineering so I opted to make lcd digital displays, and integrated touchscreen computer/audio. But you can design with any instrument cluster. (if your going for a new hip look, you may want to hit up a scrap yard, and pull some clcusters from new vehicals.
4-The method i used for designing a dash, was the same skill's used for building model airplanes. Basically you create a skeletal structure, I used alluminium stock for the frame, and mounting points for gauges/clusters etc. I used plate steele for mounting points to firewall. Once frame was built, and gauges were tested for fit, I "skinned" the dash frame. I tried a few methods, and in the end I used balsawood(old habits die hard), due to the fact that is bend on curves easy and is easy to sand. Once skinned, I sanded, used fillers, and sanded some more. Once this was done i tested fit of gauges, switches ets, and using a drill, drimmel etc, made adjustmesnts. in a few instances i had to refab some areas(no big deal, just more time). I then installed the dash into vehical and verified fit.
5-Once i was happy with fit, i removed and began to carbon fiber. I was up in the air if i would fiberglass or carbon fiber, in the end i choose carbon fiber because off look. I read up for a few weeks on all the methods of laying the cloth etc, then made a trial run on a speaker box. Either way i spent a good week, taking my time, putting the cloth and resins on. It was awesome. Once i was finished i installed gauges, puter, switches, and adjusted for fit(I should point out that from the begining stages i allowed room for skinning and glassing diameters). then i test installed in vehical and made adjustments.
6-the final stage also took me a great deal of time, but may be quick for others, that is wiring. If you are using compatable gauges then just wire up, if you are like myself, everything was custom, and thus i had to design analog to digital converters, encoders, etc, then write the software to use them.
7-in the end i won a buch of car shows locally because of the job, though to be completely honost the purpose behind doing that initial dash was I wanted MP3, computer and wifi in my old camero lol. Much is the same for the pathy im getting ready to do. Yes i know i can purchase a system now with all that, but back then it didnt exsist, besides I like the idea of pulling intoo my garage, and having my pathy computer link to my wifi and update my itunes library :P
In a nuttshell if you are a crafty guy, and like tooling, fabing, this is a fun project, but not an overnight one