A Mitzvah and obligation upon each person:
Best to personally perform all the Shabbos preparations: Even if one has many servants he is to endeavor to personally prepare [all or as much as possible of] his Shabbos needs. [This applies even if doing all the Shabbos preparations will come in expense of his Torah learning.]
Obligation to personally perform one act of preparation: Even if the person is a very prestige figure and great Torah scholar, of which it is unusual for him to purchase items in the marketplace, or do certain labors in the home, nevertheless he is obligated to endeavor to personally perform at least some of the Shabbos preparations. [Practically although it suffices for him to perform even one act of preparation nevertheless it remains a Mitzvah for him to engage in many acts of preparations, even if this will come in expense of his Torah learning. In such a case that one will only be performing one act of preparation it is best that he involve himself with the preparations of the food that is most enjoyed by him, in order so he also benefit from the Mitzvah of “better to perform a Mitzvah personally then through a messenger”.]
Performing even belittling acts for the sake of Shabbos: One is not to hesitate performing even belittling acts for the sake of honoring Shabbos, as honoring Shabbos is itself one’s reason for honor, and so did the greatest of our Sages of which each person is to emulate.
Summary-Each person is to personally help prepare for Shabbos:
It is an obligation upon every person to personally perform at least one act of preparation for the honor of Shabbos. The more preparations that one personally performs the greater the Mitzvah, and one is thus to strive to personally prepare all his Shabbos needs, even if he has many servants.
Sparks of Chassidus: 
It is written in the name of the Arizal that the sweat which one breaks due to preparing for Shabbos is auspicious for erasing one’s sins, just as are tears. Therefore one needs to exert much effort in honor of Shabbos.
When on Erev Shabbos is one to begin the preparations for Shabbos?
Begin the preparations in the morning: One is to always wake up early on Erev Shabbos in order to begin working and preparing for the needs of Shabbos in the morning of Erev Shabbos.
Add in preparations also by Bein Hashmashos: Aside for the preparation done in the morning one is to also add in preparation during twilight [after sunset].
It is a mitzvah to begin preparing the food in the early morning, and then later during Bein Hashmashos.
 Ketzos Hashulchan 70 footnote 14 and so is implied from Admur. In any event the more Shabbos preparations one personally performs the greater the Mitzvah.
 As it is better for one to personally fulfill a Mitzvah rather than do so through an emissary. This concept is a general rule which applies by all Mitzvos.[ibid]
 Mishneh Berurah 250 in Shaareiy Tziyon 9. This unlike the understanding of Ketzos Hashulchan 70 footnote 14 [and Kitzur Hilchos Shabbos 250 footnote 3], which understands from Admur that one who is able to learn is rather to do so and have another do the remaining Shabbos preparations [besides for the one which he must personally perform]. Likewise the Kaf Hachayim 251/22 and Aruch Hashulchan 251/3 rule that one who has someone else available to prepare his Shabbos needs for him, is to do have them do so if he plans to spend his time learning..
Defense of the ruling of the Mishneh Berurah in accordance to Admur: It is evident from Kuntrus Achron 2 that the above ruling applies even by a Torah Scholar, as Admur there explains that only by the Amoraim which were Toraso Umnaso did they have to suffice with one act of preparation and not more, being that they had to follow the dictum [Yoreh Deah 246/18] that learning Torah pushes off a Mitzvah which could be fulfilled through others. This wording of Admur implies that today being that the Halachic concept of Toraso Umnaso no longer applies, seemingly the Mitzvah remains upon all to increase in their Shabbos preparations even on expense of their Torah learning. Now, although following the above dictum is not limited to one who is on the level of Toraso Umnaso, and rather all Jews which are learning are to continue learning if the Mitzvah is able to be preformed through another, [as explained in Yoreh Deah ibid], nevertheless preparing for Shabbos is considered like a Mitzvah Shebegufo, in which the above dictum does not apply. [Shaareiy Tziyon ibid] This too is possibly hinted in Admur which regarding the above dictum referenced the reader to chapter 240/8, which is not the source of the dictum, as there it is explained that only by a Mitzvah Overes is one to precede the Mitzvah to serving his father, thus implying here too that by this type of Mitzvah of preparing for Shabbos [which is no less than serving ones father] one only gives up its extra preparations if he has to do a Mitzvah Overes, and the idea of Torah learning being defined as a Mitzvah Overes only applies by one who is Toraso Umnaso.
Accordingly the footnote of the Ketzos Hashulchan [70/14] which understood the M”B to be contradicting the Kuntrus Achron of Admur, is inaccurate and in truth the explanation of the Mishneh Berurah compliments it.
However based on this, that today even learning is to be pushed off, a question would arise on the ruling of Admur in 250/2 that there is no need to diminish in ones learning sessions or other activities if others are doing the preparations for them. In truth however this question would regardless apply as according to all there is a Mitzvah to personally prepare all or many of one’s needs of Shabbos, as Mitzvah Bo Yoser etc., thus why does Admur in 250/2 not require diminishing other activities even if someone else will be preparing for Shabbos on his behalf. Hence one must conclude that in 250/2 Admur was referring to the letter of the law while in 250/4 he mentions the preferred method of one doing as much as he can personally.
 There is an obligation upon every person to do some preparation for the honor of Shabbos, as is evident from the fact that many Tzadikim even though they would learn torah without stop ((תורתו אומנתו, on Erev Shabbos they would stop to do this mitzvah. [250 Kuntrus Achron 2]
 As this is required out of respect and honor for Shabbos [which is an obligation upon all, and thus at least some act of honor in preparing for Shabbos must be done by all]. [ibid]
 Meaning that at the very least one is obligated to perform one act to fulfill the Mitzvah of honoring Shabbos, which is a Mitzvah that cannot be fulfilled through another person. However to personally perform more than one act is not an obligation, but nevertheless is a Mitzvah, as said earlier, that it is better to perform a Mitzvah personally than through an emissary. [Kuntrus Achron 2]
 This follows that which was explained above that it is better for one to personally perform a Mitzvah then for one to send an emissary to so for him. See footnote there!
 Kuntrus Achron 2. There Admur explains that when the Amoraim would salt the fish, which was their greatest delicacy, they fulfilled, in addition to the obligation to assist in preparing for Shabbos, as well the Mitzvah of “Mitzvah Bo Yoser Mebishlucho”
 Kuntrus Achron 2.
 This is done in order so one also benefit from the Mitzvah of “better to perform a Mitzvah personally than through a messenger” in the item of which the actual Mitzvah of Oneg Shabbos is being performed with. [ibid]
 Thus there is no room to say “How can I belittle my honor with doing such a belittling task”, as in truth by doing so he is really gaining the respect that his honor demands.
 They themselves would perform tasks which were beneath their dignity. Rav Chisda would chop vegetables very thin; Raba and Rav Yosef would chop wood; Rav Zeira would ignite the bonfire; Rav Nachman would tidy the house. [It requires further analysis why Admur omitted the names of the Sages, in contrast to the Michaber which lists them as written in the Gemara.] This was all done to show their respect for Shabbos, emphasizing the importance it had to them, and that they were in awe of its honor. This is just like a servant who is hosting his master in his home in which case the servant endeavors to show that the master is of importance to him and he awes his honor to go out of his way and do preparations for his arrival. [ibid]
 Shaareiy Teshuvah 250, brought in Ketzos Hashulchan 70 footnote 14
 This is hinted to in the verse [Exodus 16/5] regarding the gathering of the Man of which it states that the Man was gathered in the morning of each day, and regarding Erev Shabbos the verse states “And it was on the 6th day that they prepared that which was brought”. This implies that they immediately began preparing the Shabbos foods after gathering the Man, which, as said, took place in the morning. Hence today we too begin the Shabbos preparations in the morning. [ibid]
 This is hinted to in the above verse which states “and there will be double of that which they gather each day”. This implies that the preparation should be done twice, once by morning of the 6th day and once towards the 7th day. [ibid]