When on Erev Shabbos is one to begin the preparations for 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. [This refers to prior to sunset, as after sunset it is forbidden for one to do any Melacha.
It is a Mitzvah to begin preparing the food in the early morning, and then later during Bein Hashmashos.
 Admur ibid; Michaber 250:1; Rav Chisda Shabbos 117b
 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]
 Admur ibid; M”A 250:1; M”B 250:2; Shivlei Haleket 55; Midrash Tanchuma
 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]
 Toras Shabbos 250:2; Aruch Hashulchan 250:1; Piskeiy Teshuvos 250:1