Comparison of crank based leg mechanism/draw the Jansen linkage
First of all, we need a drawing with the constants.
Theo Jansen has published the numbers on his website: 
I suggest to name the points. The naming can be arbitrary. I have used Z..S to avoid confusion with the lengths a..m.
It can be very helpful to print the drawing to scribble on.
Construction by handEdit
Now we have to think about how we could construct the mechanism by hand.
We need a non collapsible compass, a ruler and paper.
We set the origin Z to an arbitrary point. For Y, we are going down l and a to the left. The crank m can be at an arbitrary angle. Drawing the crank results in the point X.
Now at this point, we construct triangles that are defined by two points and two lengths.
In geometry, this is the SSS case (constructing a Triangle with three sides given, see also Solution of triangles#Three sides given (SSS)).
On paper, this can be easily solved using a compass.
Let us start with setting the compass to length b and then putting the compass in point Y. Then we set the compass to the length j and put the compass in point X. The crossing point of the arcs is the point W.
Note that when two points and two lengths are given, there are always two solutions, speak crossing points. Given that we already know the general shape of the mechanism, we know which one we need. But keep it mind for later.
The rest is more or less "rinse and repeat".