articulation is accomplished by sewing darts into the edges of the front panel
around the knee.
If your pattern
calls for you to hem the pants to length after sewing them, like Thru-Hiker's
wind pants project, then all you have to do is know where the knee will be and
sew the darts into the two edges of the front panel around the knee. The back
leg panel will end up being shorter than the front panel by the amount of fabric
taken up in the dart. Since you'll be cutting them to length and hemming this
won't make any difference as long as the total length of the pants still fit
If your pattern
specifies length, you can add articulation by cutting the pattern and adding
a block of paper where you want the articulation. With pants, the inseam and
outer seam of the front and back panels must be exactly the same length. The
block of paper you'll added will make the front panel's inseam and outerseam
longer, but you can make sure this extra length will be taken by the 'z' of
the articulation dart. The width of the block of paper is twice the width of
the base of the articulation dart; for a half-inch dart, you'd lengthen the
front panel pattern by adding a one inch block of paper.
You can make multiple
small darts or a larger dart and get the job done. If you're not sure what you want, try a single dart around 5/8" wide at its base and 3"
long. So for a pair of pants there are a total of four darts, two on each side
of the front panel per leg. The bigger question to ask is "do I really
need articulated knees". I haven't found them to make a significant difference
in the fit or function of pants I've made and would avoid them unless I had
a compelling reason to need them.