I begin by explaining that all the sewing materials have the
same rules as the things we use for science equipment (which can include
rubber bands, staples, etc.): they are to be treated with respect and used
only for the purpose intended. I tell them in advance that I know some
kids like to stick needles in the top layer of their skin but that is not
the purpose of these pins and needles. I know it "doesn't hurt" but I am
responsible for them and I will not risk an infection.
Next I show the pinned pieces and explain that they will be sewing on
the pencil line. I explain how accuracy is important so that all our blocks
fit together. I show the threaded needle and explain they they will not
have to thread their own (sighs of relief usually go up here).
I show the students how to start at one end of the sewing line, usually
on the right for right-handers. Then I show them the beginner's stitch.
Stab the needle down through the pencil line, pull it all the way through,
stab it up through the pencil line an eighth to a sixteenth of an inch
away, and pull it all the way through, etc. 
When you get good at that, I tell them, you can try semi-pro. Then I
show them how to push the needle down through the pencil line just a hair
and bring it back up again a short ways away and take one full stitch at
a time. 
Finally, I say, if you get good at that you can try the expert method
where you take several of the stitches on your needle before pulling it
through. 
I tell them great quilters can get eight to twelve stitches in an inch,
but I expect them to get at least six (then I accept five, shh, don't tell).
I tell them about "toenail-catchers", stitches so big you can get a toenail
caught in them and say they are forbidden.
About this time I've come to the first pin. I explain that you don't
take out a pin until you come to it, and then it comes out of the fabric
and goes directly into the "garage"-- the pincushion. In fact, any time
you stop sewing, even your needle should be "parked" in the garage.
At the end of a line of stitching I show the students how to make a
triple backstitch to tie off, instead of a knot. Three stitches one on
top of the other will hold as well or better than a knot and is much easier
for kids to do.
Finally, I explain that I will expect as many pins parked in the garage
as I've used on each pinned piece as well as a needle before they can sew
the next set of pieces.
I like to do this with no more than four kids at a time so that everyone
can get in close and see. Other students can be working on picture blocks
or some other activity.
