1. The Time to Kindle the Shabbos Lights, 4 Seifim: If there is a doubt as to whether it is dark, this is bein hashemashos (this is the amount of time to walk 3/4 of a mil after shekiat hachamah (Tur) [a mil is 1/3 of an hour less than 30 minutes; A mil is a Talmudic measure of distance equivalent to approximately 960 meters (.596 of a mile) according to Shiurei Torah.]) we do not tithe what is certain, we do not immerse the vessels, we do not light the candles and we do not establish the eiruv techumim [A halachic construct enabling a person to extend the 2000 cubit distance – approximately a kilometer – that he is permitted to walk beyond a city’s Shabbos limits. Employing an eruv techumin enables a person to walk an additional 2000 cubits (Rambam, Mishneh Torah, Hilchos Shabbos 27:1; Hilchos Eruvin 6:1-2)] (See Siman 415, Seif 2). However, we do tithe the Demai [Produce concerning which there is a question whether it was tithed or not. As a stringency, our Sages required tithes to be separated from it], and we insulate hot things and we establish the eiruv for the courtyard (And see Siman 393). It is permissible to say to a non-Jew during bein hashemashos to light a candle for the needs of Shabbat. And also it is permitted to tell him to do any work that is required for a mitzvah or it is a taxing and pressing matter. RAMA: And therefore one who accepts Shabbat upon himself an hour or two before nightfall can tell (the non-Jew) to light the candle and direct him to do other things he may need [Mahari"v]
2. There are those who say that we must add from the weekday (mundane) to the holy and the time of this addition is from the beginning of sunset, i.e., when the sun is no longer visible on the earth’s [horizon], until bein hashmashos. This period is equivalent to the time [sufficient to walk] three-and-a-quarter mil (this is approx. 60 minutes). If one desires to deem this entire period of time as tosefes Shabbos (the addition to Shabbos), he may. If he wants to add less, he may. This is provided that one will add some amount of time when it is certainly day from the mundane to the holy. The time of bein hashemashos is 3/4 mil, which is like walking approximately 960 meters or .596 of a mile before nightfall. RAMA: And if one wishes to do this earlier and accepts Shabbos upon himself from Plag Hamincha and onward, he may do so. (Tur; Agur in the name of Tosefos)[See Siman 267].
3. A person who is not knowledgeable regarding the measure (of bein hashmashos and does not know when to add to it) should kindle the [Shabbos] lights while the sun is still visible on the treetops. If it is a cloudy day, he should kindle them when the roosters sit on a beam while it is still day. If he is in a field where there are no roosters, he should kindle them when the ravens sit there while it is still day
4. After the answering of 'Barchu', even though it is still day, one does not make an eiruv and does not insulate because they accepted Shabbat on themselves. And as far as he is concerned, where they say 'Mizmor Shir l'Yom Hashabbat' (before Barchu) it is like they said Barchu, as far as he is concerned.