Is there some one you know who's a little jumpy? This instructable is going to tell you how to take advantage of that and make your victim hopefully give a little scream.
Step 1: Staring
You should probably start with this. While your victim is near or talking to you stare at something stationary, preferably a wall. This will make people feel slightly uncomfortable and prepares them for the next step.
Note: This will take a little bit of time to work and you may have to repeat it to make yourself seem permanently weird.