As a referee, I must strongly disagree with the descriptions presented here.

First of all, any particular sign should be completely separate. I.e., if you intend to signal "Attack from the left is parried; riposte arrives." you would first signal "attack from the left", then put your arms down. Then, signal "parry". Again, put your arms down. Antepenultimately, signal "arrives" to your left, and then put your arms down. Finally, lift your right arm to award the point to the right.

In practice, the neutral, arms-down position between each signal is not normally really followed; most refs just flow from one signal straight to the other. What they absolutely should NOT do, however, is signal two things at the same time.

So, with that said, here are the signals:

on the side which had the attack, hold your forearm parallel to the ground, hand flat, facing down, fingers bent towards ground.
on the side which got hit, extend your arm straight out, palm facing the fencers.
on the side which had the parry, hold your arm out, bent at the elbow so the forearm is straight up. Bring the other arm over, and form a cross.
has no signal. Go straight from parry to whatever happened with the riposte.
on the side which got hit, basically place hand at hip, swing it out, and back in, as if doing the robot.
En garde
Similar to "attack", but without the hand bent, and on both sides.
Rotate hands from "En garde" so the palms are now facing the fencers.
From the "ready" signal, swing forearms in, so palms face each other. Do not clap.
"Is no"
on the side opposite the attacker who missed, point finger at ear, swing forearm out to about a 135° angle, then swing it back in.
Exactly like "attack" but on both sides.
Nothing done
With palms facing ground, straight in front of you, cross the forearms in a swinging motion, then uncross.
Both are hit
Exactly like "arrives" but on both sides.
Exactly like parry, but without the arm crossing. On side that got the touch (in épée, possibly both).