I don't use an iPhone, I use Android. But this is what I do.
Before I leave the house, I start the tracklog app - I use GPSLogger.
https://play.google.com/store/apps/details?id=com.mendhak.gpslogger&hl=en_GB
I travel, take a bunch of photos and then return home. In the GPSLogger app, I stop the logging and upload the GPX file to dropbox (the app supports this).
I import the pictures into Lightroom, go to the Map module, load a tracklog, select all the images (using the film strip) and then choose 'auto tag photos'.
The key obviously, is that your camera and phone have to have the right time settings (and have the time timezone set). You can manipulate them afterwards, but it's much easier to get it right first time.
It's not 100% accurate, the GPS jumps around a bit, but it's good enough for me, since I use it for general locations. If I was a wildlife photographer who wanted to know exactly where they were for a specific shot, I might do a bit more due-diligence on the data.