i have a digitech multi effects pedal and i love the wah on it but it would be great and sound so much better if i had an expression pedal so i decided for my first big project i could build one. i don't know anything about building pedals at all so first of all, what materials do i need and where can i get them.secondly, any links would be great, and third, how much would this cost?

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1) you need an enclosure, this is that hardest part. Cannibalize and old broken/cheap wah wah or other such expression pedal to get it.
Smallbear have one for $40 though you might be able to get something to rip of for cheaper somewhere else http://www.smallbearelec.com/Detail.bok?no=548

or if you're feeling innovative you can turn something else into an expression pedal: http://aronnelson.com/gallery/My-DIY-Wah/WAH4

2) if possible figure out any specifications for your multieffect on what it wants. Otherwise just get a pot (I think 25kohm?) and a stereo jack, wire one end of the pot

get a stereo cable and a stereo jack, and wire up the 3 terminals the the three terminals of a potentiometer and mess around till it starts doing stuff
Is it an auto-wah?
I'm not very active here on UG currently.
I'm a retired Supermod off to the greener pastures of the real world.