how do i make a simple spy camera ? Answered

You can get any of devices from www.techasus.com to make a spy camera. I bought action camera from there, took it apart and fitted insides in to a book. That action camera can work on sound activated mode and stay on stand by for hours. The quality is footage was much better than I've expected.

Not simple.
You could just use the phone, but if that's not good enough it's not going to be simple.

L