.x files are just a format that you can save a 3D polygon model for exampel. If you mean how to make a 3D model then you should buy or use some modeling program. Many of this programs costs alot of money and many dont and some are even free. I will just give two examples because theres so many programs out there. Take a look at the 3D model forum and you'll find some discussion of whats the best.
You can download:
Milkshape 3D [url]www.swissquake.ch/chumbalum-soft[/url](Sharewear means you'll need to pay after a mouth try. Really good can export to .x)
Anim8or [url]www.anim8or.com[/url](all free. tutorial at the website. Can't export to .x you'll need a converter)
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