How to apply 4-point garbage matte in Final Cut Express?

My imported video has a row of noise at the bottom of the frame which
I'd like to mask out. From what I've read, the 4-point garbage matte
filter seems like a good tool, but the descriptions of it I've read
are too terse to be of much use. I don't know how to apply it. I'm a
real newbie at this and need point-by-point instructions. I can make
the four numbered "handles" appear, but I can't drag them; and I don't
know what figures to type into the matte window to create the kind of
matte I want. If you can help, please write me at as
well as posting here. Thanks.

 

Mattes, blech. Do this. Double-click on the clip, select the "Motion"
tab, and under "Crop", enter values for "Top", "Bottom", "Left" and
"Right". Here's what typically works for me:

Top: 1.0
Bottom: 1.5
Left: 2.0
Right: 2.0

That might not always work -- I've already found a tape that needs
something more like this:

Top: 1.0
Bottom: 1.5
Left: 1.0
Right: 3.0

....which of course you'll need to shift a bit to the right with:

Center: 8.0

....or something like that. Start with the first set of values shown
above and experiment. Those settings will probably look fine with any
tape, but you want to make sure you crop out all the noise plus any
jagged edges, but not so much that you lose valid picture area.

 

That's cropping. not matting.

You can not do articulated mattes in FCP.