There is a very good book on heads up putting called "Instinct Putting Revisited" by Bob Christina, Cary Heath, and
Eric Alpenfels and only 121 pages long. It also has drills and charts. Throughout the book it is called "IP". It also mentions of some PGA Professionals that have used the "IP" system in tournaments such as Spieth, Garica, Oostheizen, Miller, Floyd, Thorpe, and others. Some do it while practicing on the putting green before a round. I have tried it with success and amazed that putts longer than 20 feet get closer to the hole this way. I have overcome the fear of the ball and clubhead and also my mind is relaxed and my body is trusted to do the stroke. It does not feel natural or comfortable in the beginning and it does help with the "yips".