With a few electronic components and some basic knowledge of breadboarding and/or soldering, it should be a relatively simple build.
Step 1: A Brief Overview of FETs
The type of FET we'll be using for this project is an N-channel JFET.
The channel in a JFET is somewhat like a garden hose - under normal conditions, water (current) flows through it at a rate determined by its area of cross section. The depletion region is like your hand. As you reverse bias the P-N junction (squeeze the hose), the depletion region grows wider (and the channel narrower), thereby increasing the channel's resistance. Then you hit a point where the depletion region has "pinched off" the channel, effectively making the JFET a large value resistor (around a couple of megaohms from drain to source).
Unlike a diode however, the P-N junction in a FET isn't capable of handling much current, and any forward biased (Vgs > 0) n-channel JFET will likely fail soon. (MOSFETs get around this by adding a thin layer of silicon dioxide at the junction, effectively disallowing current flow).
Okay, so let's get started.
Step 2: Parts
--(3) 50k trimpots
--100k pot, audio taper.
--500k pot, audio taper.
--(3) 4.7 uF
--(3) 22 uF
--(3) J201 JFETs
--Clip for said battery
--An enclosure for the thing, if you choose not to breadboard
--(2) Jacks for input and output
--A guitar? (you could probably plug in a mic instead and try singing into it, but...)