Many men are accustomed to go the Miqweh before every Yom Tob. Those who are unable, or do not have the custom to do so, should do the following. 9 Kabs of water (approximately 3.6 U.S. gallons or 13.5 liters), need to be poured on a person. If done manually, it must be poured by someone else and there should be no interruption in the flow of the water.
Many, today, use a shower instead. When doing so, one must ensure that the same minimum quantity of 9 Kabs of water flows continuously on the person. Since one doesn't know exactly how much water flows from the shower, one should measure the appropriate amount beforehand, and time how long it takes for the correct amount of water to flow.
It is worth reiterating that this applies only to the subject in hand, that of men immersing themselves before Yom Tob. Women who need to immerse themselves to be permitted to their husbands, may not follow this method under any circumstances. The rules pertaining to that situation are much stricter, and can only be effected in a Kasher Miqweh, together with all the accompanying Halakhoth.
(See Ben Ish Hai, 1st year, Nisabim, Oth 3. Maamar Mordekhai [Eliyahu], Hagim, 21:17)
Hilchot Yom Tov, Laws of Festivals and Holidays, Halacha, Yom Tob, Miqweh Mikveh, Men immersing in Mikvah