The easiest place to start from is off Kinney Road.
As far as mapping out a loop; this is difficult as many of the trails are unmarked. I would suggest either using Maprika as it has 99.9% of the trails in Stephens or linking up someone else's gpx or Strava. I've attached one of my Strava rides that starts at the Iron Gate. https://www.strava.com/activities/1366574366
If you want a tour, don't hesitate to ask. Sometimes this is better as the locals know which trails in which direction are the most funerer. Full disclosure, my fitness isn't what it used to be, so this would be an intermediate/intermediate+ pace.