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.