The instructions are located in the extractrom.c file itself and reproduced here for clarity:-

 * REQUIREMENTS
 * This program requires a patched unshield 0.6 to correctly support newer
 * installshield cab files.
 *
 * $ wget http://bit.ly/aWIjG2 (used bit.ly link for blog due to styling issues)
 * $ tar xzf unshield-0.6.tar.gz
 * $ cd unshield-0.6
 * $ patch -p1 < ../unshield.patch
 * $ ./configure --prefix=/usr && make && sudo make install
 *
 * COMPILATION
 * $ gcc -Wall -lunshield -o extractrom extractrom.c
 *
 * USAGE
 * $ ./extractrom /path/to/RUU.exe
 *
 * If successful it will create rom.zip in your current directory.
 *

(pay some attention to the NOTES and TODO sections - at the moment the tool doesn't clean up its temporary directory...)

Example Usage:-

$ ./extractrom RUU_Bravo_O2_UK_1.20.207.1_Radio_32.36.00.28U_4.06.00.02_2_release_124865.exe
Extracting data1.cab...
Extracting data1.hdr...
Extracting rom.zip...
Done!
$ ls -lh rom.zip
-rw-r--r-- 1 kenny kenny 147M 2010-06-24 13:06 rom.zip

You can download it from here: extractrom-0.1.tgz.

Update (2011-12): This is now maintained in github at: https://github.com/kmdm/unruu/