Unfortunately, a lot of the really scary costumes are major, big-budget projects. Daleks and Cybermen are very cool, but lots of folk lack the time, money or just the space to build a major costume.
One of the most impressive recent monsters are The Silence, but they're another big prosthetic project.
What about somebody who has seen a Silent? Now, that's another proposition...
Step 1: The Context
The next time you see one, though, while you are looking at it, you remember what is going on.
At that point, the Doctor's companions started keeping a tally of how many Silence they see, writing it in the most obvious, un-loseable place they could; their own skin.
Step 2: The Actual Instructable...
Well, anything that will mark your skin will do - board marker, sharpie, ball point - but the best option is probably an eye liner pencil.
Up and down both arms, and on your face if you feel like it, mark groups of tally marks.
Step 3: The Hook
So, you wander around with your tally marks, acting absolutely normally, until somebody mentions them:
"The tally marks"
What tally...? Oh! I was just... I was counting... Actually, you know, I'm not sure what I was doing. I forget.
At that point, you gaze past your questioner's shoulder, dig your pencil out and add another mark to your arm, all without taking your eyes off the point in the air past their shoulder, not even blinking. Then you do blink, frown in a puzzled way for a moment, then carry on as if nothing has happened at all.
Thanks to Roger-X and his friend James for their modelling skills.
(If there are a group of you, one person could instead wear a smart black suit and white shirt, and carry a Silent mask [go here and scroll down for a printable download]. Every time they pop on the mask, the other members of the group start acting distracted, and add another mark to their tally. This will probably work best at a party.)