Answer:
I wouldn't waste my time building one when you can just buy one for under $100. Check the site below for some cool examples :-)
But you can get a digital camera or a video camera or whatever and put it somewhere obvious but not to obvious, then you put like a jaket or something over it, but make sure you can still see the lense. if you are gonna out a jaket over it, make sure there are other clothes around it, so it looks like ya mum has got dirty laundry or something.