He is pose-able and all of the individual body pieces come apart just like a real Lego minifigure.
Step 1: Planning
This Lego Darth Vader is made primarily from foam core, which I purchased from a picture framing shop. You can find foam core at Walmart as well as most office supply stores. I also used some brown craft paper and thin painter's masking paper, both of which I got at my local hardware store. I also used some flat single-ply cardboard, craft foam, and some other basic materials.
I based all my dimensions off of the second image shown here, which I found through a google search. I enlarged it to fit an 8.5" by 11" paper, and printed it out. From this sheet, I measured and multiplied each length, diameter, etc., by 3.5 and and drew out all the pattern pieces accordingly. (It's projects like this that make me love the metric system. Millimeters are awesome.)
I apologize that I am not including some kind of pattern for this, although I may do so in some later project. I made approximately 80 individual pattern pieces which were then used to produce the over 300 individual pieces that went together to make this Lego man. (And that doesn't include any of the trial pieces and re-do's.) So it'll be a lot of work to turn those into a PDF anytime soon!
Overall this may look quite daunting and complex, but each individual step was actually quite simple. The problem was that there were just a whole lot of steps, and lots and lots of repetition for certain pieces (like the head and the helmet). I worked on this a little at a time over the course of a month, and eventually it was done. I enjoy the creative process, and find projects like this to be very therapeutic.
To make this, there were some tools I just couldn't do without. Aside from basics like white glue, hot glue, and scissors, I used a utility knife with a snap-style extendable blade, a compass, a protractor (a round 360-degree version), a metric ruler, and an exacto blade with lots of extra blades.