There are no real materials for this spell. It'll come from
your head and energy you can get when you ground and center.
What you do is charge up, ground and center and otherwise do
what you do for a spell. Then you say:
"Enemies fly and fall"
"Circling arms raise a wall."
As you envision a wall like the one in Spiral. It's
an energy barrier that keeps everything out, and it's
invisible until someone runs against it and they realize they
can't get through. If you have the episode on tape, you can
re-watch the scene, and if you don't, that is the best way I
can describe it. Remember also the main thing about shields
as you do it: that if you can't feel it, no matter how hard
you imagine it, it's not there. You need to feel the room around
you quiet down and the feeling of protection (or lack of nakedness)
that a shield confers.
And we're done...