Simply visit each solid tick/check on the map checking in at each hotspot following the onscreen prompt. Each hotspot will have a plaque located on the floor or the wall. Once checked in a hotspot's icon will change to a hollow tick/check on your map.
If you reach 99/100 hotspots and cannot find the 100th, take care to check that the hotspot's icon isn't hidden under another icon such as a "Criminal Convoy" "Gang Hideout" etc.
Alternatively this achievement can be earned without the interruption of online invasions in multiplayer free roam - thanks to L 4Stir