First off, make both your hands into L-shapes (pointer finger straight and thumb stuck out at a 90% angle). Do this with the back of your hand towards you and your palm facing outward, towards your target (hereinafter referred to as "palm-out"). Your right hand, from your POV, will be a mirror-image L. Practice this move, as it is the building block of almost all of these signs.
The Clueless classic. Make both hands into the L-shape, then put the tips of your thumbs together and and angle the tips of your pointers outward until you make a W-shape. Display palm-out to your target.
Make both hands into the L-shape, then turn your hands palm side toward you (hereinafter referred to as "palm-in"). Cross your thumbs at the joint and angle your pointer fingers to make a W-shape. Your forearms will be crossed in approximately an X-shape. Display palm-in toward your target.
Make your right hand into the L-shape (see, I told you this was important). Put the back of your hand against your forehead and display palm-out toward your target.
Make your right hand into the L-shape. Hold at arms length and display palm-out towards your target. Note: Some people prefer to use the "loser" and "lame" signs in the opposite way, which I will admit makes more sense in light of--
Shoot the bird, with your thumb at a 90% angle and your pointer bent at the second joint only (semi-Chicago-style). Place against your forehead and display palm-out towards your target.