Read the Wikipedia article, all the theoretical information you need is in there. You'll need a broad spectrum IR light source and a photo diode along with the interferometer setup they show, although exact construction of that is a bit beyond what I know I'm sure the mirror distance is critical and linked to the frequencies used,more research on Google should help with the physical construction. It's definitely not a simple project.
The basic concept is pretty simple though, the mirrors are setup so one is moveable, the partial reflection from the moving mirror cancels out some of the light coming from the IR source so it bends the spectrum.The photo diode is measured as the mirror is moved and you'll get the spectral curve of the sample, you then use reference standards to calibrate the sensors readings, and walla a trans fatty acid sensor =) I guess it's a method used to measure trans fatty acid content of vegetable oils.