Ok
First thing's first.
Here is a link to the best guide I could find:
http://www.dcemu.co.uk/vbulletin/sho...hlight=descent

REMEMBER: Make SURE you have the FULL version of Descent installed
After following all of the steps you could have 3 possible problems:
1. error "cannot find descent.tex"
2. error: something about strings
3. The update patch doesnt seem to be working.

1. To fix this error you must extract all of the files from the dx1-rebirth folder to
X/PSP/GAME/DESCENT and not X/PSP/GAME/DESCENT/dx1-rebirthxxx
so that the 'DESCENT" has only ONE subdirectory "goemon"(only if you've installed the music patch otherwise no subdirectories.

2.Errors 2 & 3 tie together. They are to do with the update not being installed properly. You may think the update has installed successfully but it has not. Again please make SURE you have the FULL version of descent. To install the update:

1. Place "1.4a update exe in your descent directory on your computer.
2. double-click the .exe and it should create another file in that direcory called "PATCH.EXE"
3.Now here comes the tricky part: Because the dx1-rebirth pack or my DESCENT seems to not have descentr.exe I had to install the update 'ingnoring' the missing files.
So heres what I typed in. Change according to directory in which descent is installed ON YOUR COMPUTER and where the patch file is therefore located
C:\GAMES\DESCENT\PATCH -IGNOREMISSING

And there you go. If you've somehow managed to lose your copy of descent you can download the full version here: http://www.abandonware-france.org/ltf-jeu.php3?id=310

A BIG thank you to F34R on dcemu for providing a very helpful guide and of course the makers of DESCENTPSP.
If something is incorrect here please feel free to correct me.

Hope this helps