How should we best instruct the respondents to answer the questionnaire?

The primary purpose of the questionnaire is not to make it easy for the people to respond to it. The only advice we can give is that it is supposed to be difficult and you just have to select the option that describes you best and the option that describes you least. Anything else would make us part of the answering process, which should not be the case. The key is that they compare the rows, not the words. If the questions were easy (like most DISC based tools have), it would be easy to adjust your answers to the direction you want and it could not anymore measure the subconscious behaviour.