You need to get an armourer's wrench and remove your current barrel nut and delta ring.
Start by unscrewing the 6/8 screws on the rear of the new rail and you'll get the end part come free.
Put that end 'cap' onto the rifle and then screw down the new rail's barrel nut to clamp down the cap, then attach the main rail and do up the screws to keep it all together.
Hope that helps, if you're still having issues let me know and I'll take off and reinstall my DD rail clone that it's based on on Sunday and take photos to make it clearer