Poor unfortunate souls. I'm guessing you tried using the Straight to ImageReady tutorial but that cursed stone-age piece of machinery wouldn't accept your video, right? No, you didn't? Then I suggest you try that tutorial first. Trust me, it will make your life full of happiness and joy and bliss. For the rest of you poor folks, this is the place.
Basically, you can't put your video clip directly into ImageReady because it's the wrong file type or has some weird codac or something like that. So what you need to do is run your video through the video editor of your choice (we'll use Windows Movie Maker (WMM) in this demonstration, because it's the most widely available), and trim the clip while you're at it to simplify the process later on. Depending on how familiar you are with the respective programs, this process can take you anywhere from five minutes to three days. Well, maybe not three days. But something around there.

Let's get started, shall we?

First, open up WMM. If you're like me and can never find the silly thing, you usually can find it in your Start > Programs menu. Once you get there, it'll look something like this.

Found it? Good.

Now go to File > Import into Collections. Select the video file you'll be using (it needs to be a wmv or mpeg file.) For this tutorial, I'm going to be making an avie of Susan's first journey into Narnia from the LWW teaser trailer.

Then, when the clips have generated, click on the clip/s that contains the video bit that you'd like to use. Move them to the Timeline, and then split the video to fit. (If you're unsure how to do this, I'd suggest browsing the 'help' links in the scroll-bar on the left of WMM. It will tell you how to split and clip the video so you get exactly what you want, no more and no less.
Don't worry if you have small bits of unwanted frames at the beginning or the ending of the clip you want. There's no need to get all precise here...you can edit the extra stuff out in ImageReady.

So, when you've got what you want, go to File > Save Movie File. Then when the window comes up, do the following.

Save it to your computer

Then click 'Browse' and save it to the file you want.

Then click 'Other Settings' and choose DV-AVI (NTSC).

Then click Finish.

Yay! All done with that blasted business.

Now head on over here to the "Straight to IR Tutorial" and you'll find the nitty-gritty on getting your clip into ImageReady there. You'll also find that I'll have jumped from using the LWW teaser trailer to using the PC teaser trailer...oh, how marvelous is time travel! ;)

Enjoy!