Cutting door glass channel

[WhippinSaw]

I am putting in my door glass channel and I am running into the same problem as in this youtube video at 17:30 minutes.

http://www.youtube.com/watch?v=hct-aTxCiEQ

I have installed the channel into the guide making sure it engaged the clip on the guide that runs down the handle side of the door. I did this first because there does not seem to be a way to cut it to length down there.
I temporally installed the vent glass to see where I have to cut it to length.
Before I cut it, Am I doing this right? What should I cut it with?

[WhippinSaw]

I just ended up marking the channel with the vent in place, then bending the felt channel to the middle of the window opening where I could get a hacksaw at it without hitting the fresh paint. The hacksaw made a nice cut but was some what difficult to cut just holding it out in front of you. After I finished the install I found the instructions that came with the felt.