First of all, he emphasizes that Shabbat is the Mishkan (Tabernacle).
That's why Am Yisrael could stop building the Mishkan and keep Shabbat instead.
For example, up until page 6, he discusses the power of thinking & stating that you're preparing for Shabbat "in honor of Shabbat."
Doing so makes you like the Kohen Gadol before he entered the Mishkan.
Then you should (as much as you can) enter Shabbat with a feeling of reverence.
Rav Miller explains a way you can achieve that on your way to shul (page 7):
It means that even when you're walking down the street, on Kings Highway, and the Friday sun is setting you should feel like you’re walking into the Mikdash.
Don't look around at what all the ignorami, all the beheimos are doing. Don’t look in the store windows.
You’re entering now the precincts of the Mikdash of Shabbos; you're tiptoeing because for the next twenty-four hours, you’re walking through a kedusha of time.
On Shabbos they ate, drank, sang and studied Torah and were happy, nothing more.
Of course they said good Shabbos, they were friendly to each other, but they didn't gab. They didn't talk devarim beteilim. [words that are extraneous & unrelated to the Shabbat atmosphere]
How could you gab in the Beis Hamikdash?
In the ancient times when Shabbos came in, an awe fell upon the people.
Very many people became different during the Shabbos.
Even the am ha’aretz, sometimes the most crude and ignorant person, on Shabbos he refused to tell an untruth; he refused to lie on the Shabbos.
We have records of that.
The Gemara (Yerushalmi Demai 4:1) tells us that on Shabbos you can ask an am ha’aretz about his produce, “Did you separate a tenth from it?” and you could rely on his word.
During the week you couldn't be sure but on Shabbos you could trust him because eimas Shabbos al ha’am ha’aretz, the fear of Shabbos was on the am ha’aretz.
You hear the old time am ha’aretz? Halevai, we should be an am ha’aretz like that.
"We’re Building A Mishkan In Our Minds; The Shabbos Mind."
(Often said about things like turning on or off a light switch, etc.)
So the short answer Rav Miller gives is that after its initial Creation, Hashem gave the world the ability to keep going on its own steam.
The sun evaporates liquid from the bodies of water, which then turns into clouds, which then replenish the ground & bodies of water with rain, and so on.
Fruit drops to the ground, rots, leaves a seed, which then sprouts a new.
Bees & flowers...
Animals reproduce and interact with each other (pleasantly or predatorily), balancing the world.
And Rav Miller notes that we, too, are doers & creators.
The world looks like it runs by itself.
And then...(page 10):
And then we come to the Sanctuary of Shabbos and we give up all of our pursuits and take time to reflect: Is it really going by itself?
We go back to the Torah and read that Bereishis bara Elokim[In the Beginning, God created...].
The Shabbos reminds us of the fact that once there was no universe. There was just a great void.
One big zero. Ayin, nothing.
And Hakodosh Boruch Hu created everything out of nothing but His word, vayomer Elokim yehi.
Hashem still orchestrates everything down to its tiniest detail.
And enjoying Shabbat properly brings us back in touch with that fundamental reality.
And we receive unfathomable reward for just keeping Shabbat.
But we can easily enhance that awareness.
Here's one tip to enhance your awareness (page 11):
By the way, it’s good to think about that on Shabbos.
When you pass by the electric switch, if you could add a little thought: “The reason I'm not flipping the switch is because I am making a demonstration that Hashem is the Borei.”
You can’t do it every time?
OK, but a few times you could remember.
That’s why when your fingers are itching to write something on Shabbos; let’s say you have some chidushei Torah and you feel like taking a pen and writing it down, or when you feel your pockets Friday night before you leave to the synagogue because you don't want to carry into the street, you should utilize those moments.
Not just that it's ossur, it’s forbidden. Oh no! It’s much more than that!
Add the thought into it:
“I’m not writing today or I’m not carrying in reshus harabim because today is dedicated to building the Shabbos. I can’t be busy with melacha now; I have a certain amount of time now, 24 hours, when I have more important work to do, more important things to build.”
But Rav Miller emphasizes doing a mitzvah with daat (mindfulness) brings you the crown of daat.
The more daat, the more beautiful your crown.
And while maintaining a mind entirely focused on Shabbat the entire Shabbat is the ideal, Rav Miller stresses that even doing this for 1 minute on Friday night and then another 1-2 minutes on Shabbat day holds tremendous value in Shamayim.
He recommends simply thinking something like, “Hashem made the world out of nothing! Yeish m’ayin!”
(This is discussed in greater detail on page 10.)
A major jewel of Shabbat is acquiring one's mind.
You can acquire this 1 minute at a time.
Cultivating Your Crown
He provides interesting details, including ancient history and how even the Roman government supported the Jewish attitude in this area of Jewish Law.
Basically, it's like someone practicing medicine without a license. Even when Jews don't know or understand the Shabbat laws & customs, their very Jewishness provides them with the Heavenly "license" to observe Shabbat.
I can't end this any better than the concluding words of Rav Miller himself on the topic:
Someday it will turn into a real crown, a crown of splendor.
You’ll sit in Gan Eden one day at that great eternal banquet and the crown of wisdom that you gained in this world by thinking on Shabbos will be upon your head forever and ever.
That’s the great success of the one who knows that Shabbos is a Beis Hamikdash.
And don't forget to check out the very easy & simple yet powerful Practical Tip on page 17!