You can set out to find the Network Relays after you have unlocked the Enigmas side mission, which has been described in the Most Wanted chapter. I recommend that you start searching after you have activated all seven of the comms towers described above, because the locations of the active relays will then be added to the world map and, as a result, you will be easily able to mark them as your destinations.
In each of the main city district, there are 10 network relays to destroy. I recommend that you use the above map here, as well as the side missions menu (Enigma). Destroying each one of the relays is rewarded with 2000 experience points, and destroying the entire network of relays (10 relays) in a given district, is rewarded with a bonus of 15000 experience points.
Note do not worry if a group of relays temporarily disappears from the world map. This is the manifestation of one of the minor glitches in the game. If this is what happened, you can still destroy the relays. You will only have problems marking them as your destination on the world map.
After you reach each new relay, you need to do the same thing, i.e. select the batarang from the inventory and use it to destroy Enigmas device. Note if there is a group of enemies around the relay, and you do not want to fight them, land on some ledge above them and take advantage of the considerable range of batarangs.