Washing the hands:[1]

Sitting versus standing:[2] Mayim Achronim may be performed in a sitting position.

Amount of times to wash the hand?[3] One is required to pour the water over his hands only one time.

Up until what part of the hand must the water be poured?[4] One is required to wash the hands until the second joint [and by the thumb until the first joint[5]].[6] [However, according to Kabala, one is to wash the entire finger, up until the third joint and by the thumb up until the 2^nd joint.[7] The Chabad custom is to wash only the tips of the fingers.[8] One is required to wash both hands. It does not suffice to pour the water on one hand and then rub it against the other hand.[9]]

Pouring versus dipping?[10] One is not to dip his hands into a vessel of water but is rather to pour the water onto his fingers.[11] [This applies even if the vessel contains a lot of water, as according to Kabala one is required to pour the water onto the hands.[12]]

Lowering the fingers?[13] One is required to lower the tip of the fingers while washing.[14]

Drying the hands:[15] Some Poskim[16] rule that one is not required to dry the hands after performing Mayim Achronim. However, other Poskim[17] rule that this applies only to those being Yotzei Birchas Hamazon from another. However, one who is reciting Birchas Hamazon [on behalf of himself or the listeners[18]] is required to dry the hands. Practically, one may be lenient in this matter, and not dry the hands after Mayim Achronim even if one will be leading the Bentching.[19]

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[1] See Ketzos Hashulchan 43:2

[2] Admur 181:6; Michaber 181:6

[3] Admur 181:4; Rashal Chulin 8:10; Mordechai Brachos Remez 207

[4] Admur 181:4; Michaber 181:4; Rashba Toras Habayis Aruch 6:1 in name of Ravad Tamim Deim 66

[5] M”B 181; Ketzos Hashulchan 43 footnote 8

[6] The reason: As the food does not reach above the second joint. [Admur ibid; M”A 181:4]

[7] Shulchan Arba, brought in Beis Yosef 181; Arizal; Siddur Rashash; Kaf Hachaim 181:17; See Piskeiy Teshuvos 181:8 

[8] Hayom Yom 4^th Kisleiv; Sefer Haminhagim p. 22; See Darkei Chaim Veshalom 303; See M”B 181:10 who records such a custom and negates it! Likewise, the Ketzos Hashulchan 43 footnote 8 and Piskeiy Teshuvos 181 negates it

The reason: Seemingly this custom is an extension of the general Ashkenazi custom recorded in Admur 181:9 to no longer require Mayim Achronim, as the reasons no longer apply. The only reason, however, that remains today is that based on Kabala, and accoridng to the Kabalistic reason, the less water one uses to wash the better, as the water goes to the side of the Kelipos. [See article of Rav Y.S. Ginzberg on this subject, printed in Hiskashrus]

[9] Ben Ish Chaiy Shelach 9; Piskeiy Teshuvos 181:9

[10] Admur 181:5 in parentheses; M”A 181:1; Rashal Chulin 8:10

[11] The reason: As the entire purpose is to wash the toxins off one’s fingers, and when one washes them in a bowl of water the toxins remain in the water, and remain attached to one’s fingers. [See Admur ibid]

[12] Kaf Hachaim 181:9

[13] Admur 181:5; Michaber 181:5

[14] The reason: This is required in order so the toxins that are on the hands fall below. [Admur ibid; Michaber ibid] so the Kelipos fall downwards.

[15] Admur 181:7; Michaber 181:8

[16] 1^st opinion in Admur ibid; 1^st opinion in Michaber ibid and 173:3; Ravad in Tamim Deim 66

[17] 2^nd opinion in Admur ibid and Michaber ibid, as explained in Rashal, brought by M”A 181:6; Rambam Brachos 6:20 that one is to dry the hands; 7:13

[18] Kaf Hachaim 181:22

[19] Admur ibid; Michaber 173:3

Other opinions: Some Poskim conclude that the hands are to be dried. [M”B 181:19; Shulchan Hatahor 181:4]