In order to use your hacking skills in each district of Watch Dogs, you first need to gain access to the server room in that area's Control Centre. Once a backdoor to the network is installed, the locations of any ctOS Towers in the district will be revealed--hacking into these towers will unlock safehouses and mark points of interest on your map.
The route into each Control Centre and Tower is a puzzle, and often involves manipulating camera feeds and machinery to reach your destination. This guide will show you how to complete all of them and reveal every item on your map.