For this achievement i keep hearing people say it has to over all seven islands which is wrong. As an added note you need level 5 agility in order to unlock the wingsuit. All you have to do is fly over the three main areas from the first game. Los muertos, shi gen, and volk. You do not need to fly over agency island trust me i have done this several times and with a bunch of people.
The shortest route is to start from the very end of volk, fly through shi gen and into los muertos, i have a picture of the route ill upload. It is worth noting that you can take an even shorter route which eliminates a few seconds of gliding). This route starts the same but cuts off the northernmost island from the route by flying directly across the water after reaching the first shi gen island into los muertos. However, this route sometimes does not unlock the achievement. if this is the case just use the route shown above. This route works for both City Glider and the Squad City Glider achievement. With multiple people i suggest all jumping out of the same helicopter over the starting point to make sure your timing is good, also the pilot of the helicopter should jump out as the helicopter is moving up so as not to get killed by the blades, which i have done a million times.