How can one be makabel Yom Tov? Unlike on Shabbos, where there are certain parts of davening that can be said, I’m not aware of an equivalent with Yom Tov.
Kabolas Yom Tov for women it is when they light the Yom Tov candles (if they are lighting before Yom Tov). This is why women do not extinguish the match after lighting their Yom Tov candles, rather it is placed down in a way that it will go out by itself. Women who are lighting after Yom Tov starts, and all men, should say “I am mekabel Yom Tov” a few minutes (and preferably half an hour) before shkiya. It is important to note that it is a special mitvza d’orayso to add to the time of Shabbos and Yom Tov.
BIur Halacha 526-1 D:H Safek, M:B 261-21,22, Shmiras Shabbos Khuilchoso 44-9, 46-2