Step 1 - Visit THIS TOPIC on windowsmoviemakers.net and download Rehan's Shader TFX Plug-In. Be sure to get the correct version for 32 bit or 64 bit windows. You could purchase RehanFX if you like and if you plan on masking a lot. But the 14 day trial works for experimenting. I have the full version.
Step 2 - Install RehanFX, make sure movie maker is CLOSED. Once its installed you're ready to go!
Step 3 - Open movie maker, and select the clip that will be the mask, and the clip to be in the background.
Step 4 - You don't need your background clip yet but its good to know what you will be doing with your mask. Now, slow down your mask to x4 slower (Slow Down: Half x 2) and go to the beginning of your clip.
Now the time consuming parts begin!
Step 5 - Take a screenshot using "take picture from preview" (in tools I think) for EVERY FRAME until the end of the clip. You could end up with 100 + depending on the length of your clip.
Step 6 - You no longer need movie maker for a while, so close it and open up your image editing program. This is the worst of it. You now must open up and individually colour the background behind your character brightgreen/blue (green or bluescreen colour of course) for each frame/screenshot. You can do this any way you like, the result is the same. Be very careful. I would recommend photoshop, but if you're hardcore you could always try using paint...
Step 7 - All done? Dying? Right. Now import all those screenshots back into movie maker. They are pictures so ensure that in settings the picture duration is set to as little as possible (I think its about 0.125 of a second) and place each screenshot, in order, onto the timeline. Just place them, that is all. No editing required!
Step 8 - render.
Step 9 - Once rendered, import the movie and speed up your clip to its original speed. It can sometimes look choppy/uneven but its a learning curve, it will get easier and neater as you practise.
Step 10 - Mask! Place your mask over your background clip and use either greenscreen or bluescreen transistion! You will have to experiment yourselves with the bluescreen/greenscreen strengths and whether to use A or B but this is the easy part.
And you're done, congratulations upon masking with Movie Maker!
Hope everyone understood this tutorial o.o