You first make a new "Single View" project. This starts out with a Storyboard, but as Xcode 6 has removed the option for creating an Empty project, we will just work with it from here.
You then create a new file for this project, go into the "User Interface" category and select "View". I would name it the same name as your original ViewController as it will replace the storyboard we are about to delete from the project.
Once the XIB is created, you will want to select it and set the "File's Owner" to point to the "ViewController" class that you want this XIB to link with. That is done by going into the Identity Inspector of the File's Owner of the Xib, and changing the default of NSObject to the class name of your view controller.
Once done with that, you want to go to the Connections Inspector to link the view of the File's Owner to the view of the XIB. Just click the little circle across from "view" and drag it over to your view to connect it. You should then have a connection between view and View.
Now the important parts. Go into your project Target, under the "General" tab. There is a subsection called "Deployment Info". In that subsection there is a field for "Main Interface". This field should be showing the name of the storyboard. You need to delete the value shown in this field, so that the Main Interface is left blank.
Then go into the App Delegate and set the root view controller of your window like you have been for previous versions of Xcode. Once that is done you should have a running app using your XIB, and you can delete your storyboard from the project without any adverse affects.