When should one set up the Blech before Shabbos?
Biblically, there are no restrictions involved in setting up one’s food on a flame before Shabbos, although the Sages enacted a number of restrictions. First off, they obligated that one may only leave food on a flame if either the flame is covered, or the food is at least half cooked. This is formally known as the Shehiyah restrictions. In addition, according to some Poskim, they also obligated that the Blech be set up a certain amount of time before Shabbos, so the food is hot before Shabbos begins. It is this latter aspect that will be discussed below:
The law: All foods that do not have the Shehiyah restriction apply to it [i.e. half cooked or the flame is covered, such as a typical Blech or electric plate] even minutes before the entrance of Shabbos. However, there are opinions who rule that if it is so close to Shabbos that if the food in the pot were to be cold then it would not be possible to heat up before Shabbos, then it is forbidden to return it to a fire [or Blech or electric plate], even if now the food is hot, unless all the Chazara conditions are fulfilled in a way that would thus permit one to return this food even on Shabbos itself. [Meaning, they hold that from before Shabbos, when there is no longer enough time to heat up the food if it were to be cold, then all the restrictions of returning a food on Shabbos begin to apply.] Practically, the custom is to be completely lenient like the first opinion, however in a situation where there is not much need for one to do so, it is proper to be stringent to not place the pot on the fire if there is not enough time for it to heat up before sunset, unless the Chazara conditions are fulfilled. [This ruling is in reference to an oven or fire, however by a Blech, and certainly by an electric plate, there is even more room for leniency. Nonetheless, it is proper to be stringent even in such a case.]
If there is enough time for the food to be heated before Shekiah, but not before one accepts Shabbos: If when one comes to return the food to the flame there is enough time to heat it up before Shabbos [meaning Shekiah] but there is not enough time to heat up before he personally accepts Shabbos, then according to all opinions there is no prohibition involved to place the food on the fire without the Chazara conditions being fulfilled, so long as there are no Shehiyah prohibitions involved.
From the letter of the law, one may set up the food on his Blech or electric plate any time before Shabbos begins. Nonetheless, it is proper to be stringent to set up the food on the Blech and electric plate with enough time for the food to heat up before sunset, if the food were to be cold. If the food is being brought directly from the flame on which it cooked, onto the Blech or electric plate, then this may be done even moments before Shabbos according to all.
Practical ramifications of the above stringency:
1. Food on the Blech/electric plate: One should not set up the food on the blech very close to Shabbos. Rather, one is to do so early enough so that if all the foods were cold they would be able to heat up by the time Shabbos begins. At the same time, one should to be careful once the food is placed on the blech that it not be removed before Shabbos and then replaced unless all the conditions of Chazara are fulfilled. One may always bring a food directly from the fire onto the Blech, even moments before Shabbos.
2. Cholent in Crockpots: When returning a cholent pot to the crockpot before Shabbos, one should make sure it is done early enough for it to technically be able to heat up before sunset, if it were to be cold. Being that crock pots are slow cookers, therefore, practically this may need to be done hours before Shabbos. Thus, one should either a) return the cholent to the crock-pot before this time or b) not remove the Crockpot without all the Chazara conditions within this number of hours before Shabbos.
Ø Practical advice: In a situation that one forgot to set up the blech or crock pot with enough time before Shabbos for the food to get hot, he can still abide by the stringency by doing the following: He is to heat up all his foods on a high flame and then transfer it with all the conditions of Chazara onto the Crockpot and blech/electric plate.
Does the above stringency apply even to food being placed on a covered fire for the first time?
No. It only applies to food which was on the fire and then removed, and now one wants to put it back on. Thus, one may place uncooked food any time before Shabbos on a covered flame according to all opinions.
If one places the food on a high flame, does he have to leave it on the high flame until it heats up, or can he lower the flame even before it gets a chance to heat up before Shabbos?
Some Poskim rule that if the pot was placed on a fire that could heat it up before Shabbos, then one may afterwards lower the fire, even before it has heated up.
How hot must the food reach before sunset?
 See Admur 253/22-23; Rama 253/2; M”B 70-78; Shabbos Kehalacha Vol. 1 page 379-381; Piskeiy Teshuvos 253/22
 First opinion in Admur 253/22 “A cooked food may be returned to a fire before Shabbos, even inside a freshly lit Tanur of those days. This may be done even minutes before the entrance of Shabbos, being that the food is already cooked and thus does not have the Shehiyah restriction apply to it.”; Rashi; Ran 15b; Beis Yosef 253; Rama 253/2 in name of Yeish Makilin
 Second opinion in Admur ibid; opinion in Rama 253/2; Tosafos Shabbos 36b; Rosh; Olas Shabbos 253/2; Menorah Hatehora 253/29; Tehila Ledavid 253/16;
Other Opinions in understanding the second opinion: Some Poskim rule that the above opinion is only stringent regarding the requirement to cover the flame before Shabbos if there is not enough time to heat it up by the time Shabbos begins, however all the other restrictions of intention etc. are not needed even according to the stringent opinion. [Gra; Tosefes Shabbos; Mishneh Berurah 253/72 rules like this latter opinion; See Shabbos Kehalacha Vol. 1 page 381]
If the food is not fully cooked: If the food is half cooked but not yet fully cooked it is disputed in Poskim as to whether it must still be placed on the fire with enough time to heat up before Shabbos according to the stringent opinion. [The Magen Avraham mentions in his 1st opinion, that even according to the stringent opinion, if the food is not fully cooked, it may be placed on the fire even close to Shabbos, as in such a case there is no worry a person may come to do so on Shabbos. This follows the ruling of the Tur and Rosh. However, in his second opinion he mentions the Smak and Hagahas Maimanis who prohibit even if the food is not fully cooked. Based on the rules of the Codifiers, the Magen Avraham himself holds like this latter opinion, and so is implied also to be the opinion of the Alter Rebbe. It requires further analysis as to the ruling of the Mishneh Berurah.] Practically even if the food is not fully cooked one is to be stringent like the stringent opinion. [Shabbos Kehalacha Vol. 1 page 379-381]
 Not cook. Meaning, as long as it could heat up before Shabbos, even though that to cook it, if it were theoretically to be uncooked, would take much longer, it is still valid even according to this opinion. [Shabbos Kehalacha Vol. 1 page 379]
 This understanding of the second opinion [Tosafus and Rosh] as written by Admur follows the ruling of the Olas Shabbos 253/2; Menorah Hatehora 253/29 and is implied from wording of the Rama “just like Shabbos itself.”; However, as brought in previous footnotes, there are Poskim who argue on this approach.
The reason: Their reasoning for requiring this is: because of a decree that [if this were to be allowed then] perhaps one will come to return a food even on Shabbos itself. [As if one were to be allowed to allow the food to heat up on Shabbos, even though he placed it there before Shabbos, then perhaps one will not differentiate between when the food is placed there and come to allow to place the food there on Shabbos itself in order to heat up, without fulfilling the Chazara conditions.] [Admur ibid]
 Lit. Good
 Admur 253/22; Rama 253/2
 Being that even on Shabbos itself there are opinions which permit one to even initially place food on it. Thus, certainly here when the issue discussed is only a stringency there is room to be more lenient.
 Admur 253/22; M”A 253/26
Shabbos automatically begins at sunset, whether or not one has consciously decided to accept Shabbos. Although, one who wants, may accept Shabbos earlier then sunset. Thus the novelty here is that as long as there is enough time for the food to heat up before sunset, he may place it on the oven according to all opinions, even if he will be accepting Shabbos earlier than sunset and the food will not be able to be heated by that time. [Magen Avraham 253/26]
Other Opinions: Some Poskim rule that that according to the stringent opinion, the food must be able to warm prior to one’s personal acceptance of Shabbos. [Rama 253/2]
 Shabbos Kehalacha Vol. 1 page 379
 The reason: The reason for this is because on Shabbos one will never come to place food for the 1st time on the fire. Thus there is no reason to be stringent that before Shabbos one may not place food on for the 1st time if there is not enough time to heat it.
 Rav Shlomo Zalman Aurbach brought in Shemiras Shabbos Kihilchasa note 185, and Shabbos Kehalacha page 379
 Minchas Shlomo 2/34; Piskeiy Teshuvos 253/22