So, I put some thought to the subject.
Looking at Shema, we start by joining the community of Jewish (Shema Yisrael), and then proclaiming that despite our disparate perceptions of Him, Hashem is one and unique. This is an awareness of G-d’s uniqueness and power. True of a melekh or a mosheil, although here we’re actively acknowledging it. We accept the fact of Hashem’s rule.
And then, before the list of pragmatic mitzvos for keeping this message an active part of our day, we are told to “Love Hashem with all your heart (kol levavekha), all your soul, and all your resources.” Willingly bowing to that rule. This is the step of which we’re speaking, the shift from realizing Hashem is Mosheil to accepting Him as our Melekh.
Chazal comment (and quoted by Rashi) perhaps on the word “kol”, perhaps on the use of the two-veis word for heart “levavekha” rather than “libekha”, that this is with both of our inclinations — our good inclination and our evil one.
… veyishtachavu lefanekha kol haberu’im,
veyei’asu kulam agudah achas la’asos Retzonekh beleivav shaleim,
kemo sheyadanu, H’ E-lokeinu, shehashalton/shehashilton lefanekha…
… and all those who were created will bow before you, and they will all be made into a single union to do Your Will with a whole heart. For as we know, Hashem our G-d, that the rule/scepter is before You…– Amidah for Yamim Nora’im
Bowing before Hashem because we acknowledge His rule is obvious. However, note again that this global union of worship is “with the whole heart”, a two-veis heart. Both inclinations. This to is because we know that He rules. But how does that cause us to engage our baser inclinations?
On Shabbos we say, “Yismekhu beMalkhusekha shomerei Shabbos veqor’ei oneg… — They shall rejoice in Your Kingship, those who keep Shabbos and call it pleasure..” It’s not enough to keep Shabbos. To be happily a subject of Hashem as King, we must find it an oneg, a pleasure.
It would seem that qabbalas ol malkhus Shamayim involves accepting the idea that following His plan is what is best for you life. Not just fulfilling the mitzvos, but seeking to do so beleivav shaleim and with qeri’as oneg.
How does one do it? I must start with the first mitzvah that I don’t do and think I can. And with the first mitzvah I do begrudgingly and search the sources and the experiences it brings me to find its beauty. Then the second…
That is working toward the day when our teshuvah is rewarded, and “vehayah Hashem leMelekh al kol ha’aretz — Hashem will be Melekh over the whole world.” Bimheirah beyameinu, amein!