Tools are a device used to fix something. Rules are a structure or boundary given to a situation allowing navigation in a certain way. Sometimes people get confused trying to use them. Awareness should be brought to the situation to gain understating on which part of the practice is a tool or rule and how to go about using it. Sokuzan breaks down and explains the difference and how to see the difference.
“Not Two” is a way of addressing ones presumptions that there is a lot of individual things, which is referred to as the 10,000 things. There is a hidden unity, though. Sokuzan teaches on this unity.
When establishing an awareness practice one can attempt this without a teacher. However, this often leads a person to a dead end or to what Chogyam Trungpa Rinpoche calls a Spiritual Supermarket. Sokuzan discusses further in regards as to when you may need a teacher.