Constant Mitzvas #3: Unification of the Divine (Achdut Hashem)

Rav Weiss continues his explanation of the six constant mitzvot. This week, #3 - How to always think about the unification of God - Achdut Hashem.  What does true unification mean? How do we perform this Mitzvah?  How does this Mitzvah help us understand the Divine?