Omit Tachanun at Mincha erev Shabbos. Replace Al Naharos Bovel with Shir Hamaalos when eating after chatzos. (ShA1 – MB1)
After plag hamincha you can light nairos, accept Shabbos, and daaven Maariv. Just make sure you daaven Mincha before plag. If you are with a minyan and they daavened Mincha after plag, they can be maykil to daaven Maariv during bain hashmashos but not earlier. Remember to repeat Krias Shema. (ShA2 – MB3)
After Maariv, you are allowed to say Kiddush and eat the Shabbos meal before dark. L’chatchila, you should draw out the seuda and have a kzayis to eat after dark. (ShA2-MB5)
Do not begin the meal within a half hour of z’man Krias Shema. Wait until dark, repeat the Krias Shema without the berachos, and then begin the meal. Do not rebuke those who wish to be maykil, they are relying on the Taz that Krias Shema was fulfilled already. (MB 6 – ShaH11)
The ending of the Hashkivaynu changes for Shabbos. If you forgot and said the weekday ending, correct it toch keday dibbur, otherwise you do not need to repeat it correctly. (ShA3-MB9)
The minhag is to recite “V’shomru v’nay Yisroel…” after Hashkivaynu. On Yom Tov “Vydabber Moshe…”. On Rosh Hashanah “Tik’u…” and on Yom Kippur “Ki vayom hazeh…”. (MB9)