Having completed the head movement, the next step is to create the mechanism that swings the legs back and forth. The existing legs are too short and light in weight to swing naturally by themselves so they'll need a little help. 

I started by creating a box to house the mechanism, on top of the box are four tabs that locate the Paper Pop into position. 

Looking at the underside of the box; given that I'm going to be hanging a heavy pendulum from the top of the box I fitted a triangular section tube across the full width of the box top, this makes the top of the box surprisingly strong. Behind that I attached another triangular piece to which I'll attach the pendulum.

The mechanism I used is fairly straightforward. I've linked together a couple of bell cranks using a single linkage bar. The notched piece on the linkage bar locates on the pendulum which is not shown in the picture above but is hanging down above the linkage and swings left to right in the picture above.  When the pendulum swings the linkage bar is moved side to side, this in turn rotates the bell cranks back and forth so moving the triangular pegs. 

I've attached a couple of linkages to the tops of the triangular pegs. These are what drive the legs. You'll see that as one leg move forwards the other moves back and vica versa.

From the back of the box you can just about see how it all fits together. I added some coins to the end of the pendulum to give the swing a bit of inertia. They're not on the picture but I've marked where they go.

It is then simply a matter of adding my prepared Warhol and gluing the linkages to the legs.

I cut a hole in Andy's TV box for the linkages to pass through. I've cut the legs at the knees and added paper hinges, these are more flexible than the original card so the legs are free to swing easily. Finally I've change the feet so that they stick straight out.

The finished mechanism works rather nicely. By happy coincidence the head swings at around the same frequency as the legs.

Next step, a bit of colour, then instructions and the download!