The following paragraphs come from the chapter on death in
Our Existential Predicament:
Loneliness, Depression, Anxiety, & Death.
This chapter is called
"An Existential Understanding of Death:
A Phenomenology of Ontological Anxiety".
This section on Authenticity is 7 pages long.
The following quote is the first one-and-a-half pages of that section.
From pages 226 and 227 of Our Existential Predicament:

VI. ONTOLOGICAL ANXIETY AS THE IMPETUS FOR AUTHENTICITY

  A. Conformity, Inauthenticity, Lostness.

     We were born into a world of quiet conformity.
Initially everything we do and say and think and believe
have been done and said and thought and believed before.
The activities we regard as worthy of our time and effort (learning, work, play),
the ultimate values and meanings we pursue (achievement, love, children),
and the particular styles and forms thru which we pursue these goals
have all been provided by our various human cultures.
How different our lives are from the lives of ancient 'cavemen'!

     Unless we find ways to wrest control of our own lives from society,
all of our decisions will continue to be made for us
by the unnoticed forces of the cultures in which we live.
We may not be told which spouse to 'choose' or which job to take,
but how free are we to reject both marriage and work as basic styles of life?
How have we been carried along so successfully by culture without noticing it?

          'They' even hide the process by which 'they' have quietly relieved us
          of the 'burden' of making choices for ourselves.
          It remains a complete mystery who has really done the choosing.
          We are carried along by the 'nobody', without making any real choices,
          becoming ever more deeply ensnared in inauthenticity.
          This process can be reversed only if we explicitly
          bring ourselves back from our lostness in the 'they'.
          But this bringing-back must have that kind of being
          by the neglect of which we have lost ourselves in inauthenticity.

          [Martin Heidegger Being & Time, Macquarrie p. 312-313; Stambaugh p. 248; paraphrase]

How can we bring ourselves back from our lostness in conformity?
What have we neglected, which has allowed our culture to absorb us?
How can we re-possess our lives, wrench ourselves away from the 'they'?

          B. Who Am I?

     But if we notice our conformity, inauthenticity, & lostness,
perhaps we have the possibility of emerging from our cultural cocoon
and creating lives that we clearly own.
Initially we are creatures of our genetic make-up and cultural conditioning.
And if we do not notice our conformity and find ways to retrieve our beings,
we will remain in our culturally-given, inauthentic selves all our lives.

     However, in addition to being products of human culture,
we are also our powerful and pervasive internal threat-to-being.
On this foundation, we can begin to construct our Authentic Existence.

         C. How Do We Become More Authentic?

     What can reverse the process of sinking deeper and deeper into the 'they'?
How can we extract ourselves from our conformity, rise above our enculturation?
How is it possible to become more whole, centered, & integrated
in a world that prevents precisely these qualities from emerging?
Beginning as conformists whose 'decisions' have already been made by culture,
how can we become more free, unified, & focused?

     Our Existential Predicament—perceived, perhaps, as ontological anxiety—
is the rope by which we can climb out of the pit of inauthenticity;
it is the handle by which we can grip our own beings.

     First we must acknowledge our ontological anxiety.
This includes peeling away the protective evasions we have so cleverly woven
to protect ourselves from the deepest truth of our being.

     Once we have revived our ontological anxiety, we must keep it alive,
not allow it to die away into comfortable obscurity once again.
Instead of letting our being-towards-death fade back
into the diversionary small-talk of the 'they',
we must focus our lives around this 'threat'.
Then our ontological anxiety can become the light of our being
—purifying, refining, & integrating
our otherwise diffuse, preoccupied, & fragmented existence.
In the light (or in the shadow) of this constant internal threat-to-being,
we are empowered to choose our Authentic projects-of-being
—those basic endeavors that correlate best with our ontological anxiety.
Returning to this deepest truth of our being can bring us back to ourselves.

    If you would like to read this whole chapter
(also published as a separate book),
you have several options:

Two different editions of the book:
An Existential Understanding of Death:
A Phenomenology of Ontological Anxiety.

The fifth edition of the larger book in which this chapter appears:
Our Existential Predicament:
Loneliness, Depression, Anxiety, & Death.
If you wish to quote from the paragraphs above,
they come from the fifth edition, 2006.
Section C begins at the top of page 227.
The following is the best way to cite this whole passage:
James Park Our Existential Predicament: Loneliness, Depression, Anxiety, & Death
(Minneapolis, MN: Existential Books:www.existentialbooks.com, 5th edition—2006, p. 226-227)

    Another book by James Park also has a whole chapter
on Heidegger's version of Authentic Existence.
The book is Becoming More Authentic:
The Positive Side of Existentialism.
Chapter 17 is "Martin Heidegger:
Confronting Existential Guilt and Death",
pages 70-78.