Note: In order to use the GPS tracking feature on your iPhone your location services must be switched on.
Here is an article that takes you through the steps in doing this - Ensuring Location enabled - iPhone
Step 1: Select Jobs
Navigate to Jobs then select the Job you wish to record On-site data.
Step 2: Select On-site
As you can see below, you should now get notified that On-site data is recorded.
Step 3: Select On-site history
To see your recorded data on the map scroll down to the bottom and select On-site History and then select your new On-site data.
As you can see below, you are now sited on the map at the Job address.
If you wish to see the GPS tracking feature on your desktop then follow the steps in this article to track your users - GPS Tracking - Desktop view