• navigate to this page on your n900
• click on the big "Maemo Community Update CSSU" banner in the right column (it's at the very top, below search bar and council election banner)
• on that page, you can choose if you want "stable" or "testing" (with more experimental stuff) - just click on one of the links at the top, Hildon Application Manager will open and begin instalation
• on the forementioned page, exactly here: http://wiki.maemo.org/CSSU#Installation you have even a video tutorial
• TA-DAH!

You may also want to try CSSU-Thumb (even more experimental then testing, but much much faster and, as others have reported and i can confirm - it works ). To get to know more about CSSU-Thumb, visit its thread: http://talk.maemo.org/showthread.php?t=84829 and read first post, there is section "How to install it?" in the first post which will tell you what to do (basically it's the same as installing cssu stable and cssu testing - you navigate to one page, click on a link, and then maybe run one program on your n900 and ta-dah! it's done)