You must have this mod to play (also include a portal gun model)
http://www.mediafire.com/?3uc96jdifsr8skq

RULES:
-You can only create a portal on a white wall.
-You can't create a portal on any other wall (different color, gates, fences,...)
-You can't create a portal on the ceiling. (the roof)
-Please follow the rules!
-And the most important thing is...DO NOT CHEAT!

Walkthrough:
Level 1-4:
http://www.youtube.com/watch?feature=player_embedded&v=BgAeqT2A2WQ
Level 5-8:
http://www.youtube.com/watch?feature=player_embedded&v=EwrQ-RaJD-8

Mission topic:
http://www.gtaforums.com/index.php?showtopic=512761