You will want to play as "New Reno". Then you will go down a set of focus tree options

You will need 3 of the following focus tree requirements to be fulfilled.

• Calling in Old Allies ( Mandatory )
  
• Sgt. Granite's Defence Company
  
• Enclave Technician
  
• Sanctuary City

After completing 3 of the 4 requirements. You should get a popup. After clicking the only option. You should now become the Enclave!

To finish 3 of the focus tree requirements. It will take 627 days.

I suggest going down "Who's the Top Dog" tree first, then down "Merc Military". Go to "Sgt. Granite's Defence Company" then after Sgt. Granite's Defence Company. Go after "Enclave technician".