How are Kohanim supposed to position their hands and in what shape during Birkhat Kohanim? Answer with most opinions backed by sources wins.
Arms at shoulder height
Palms toward the ground
Right hand a drop above the left
5 windows are created: splitting the hand between 2 fingers and 2 fingers makes a window on each hand; splitting between the forefinger and the thumb makes another window on each hand. You now have 4 windows with each hand looking like Mr. Spock's. The 5th window is created through touching thumbs, with the right thumb on top of the left thumb.
Shulchan Aruch O.C. 128:12 Reasons are brought in the Mishna Berurah there.
The Ben Ish Hai Year 1 Parashat Tetzaveh brings a Machloket as to whether the correct method is as Maran describes in the Shulhan Arukh or whether it is according to the AriZal(Sha'ar HaKavvanot 40A Pri Eitz Haim sha'ar 10 chapter 4) that states that the hands are supposed to be at the sides of the head. The Ben Ish Hai concludes that the hands should be held in accordance with the Ari with palms facing in toward the head.
Rav Ovadia Yosef Shlit"a rules in Halicot Olam volume 1 page 207 rules(unusually) against the Shulhan Arukh in favor of Ben Ish Hai, with the exception that palms should face forward.
Rav Avner Apjin in his sefer Divrei Shalom O.C. 182 rules like Rav Yosef, while also bringing several other minor variations from the words of the RaShaSh, the Torat Hakham and several others.
The Gra holds that you spread out all the fingers and don't do the "windows" thing (Maaseh Rav 167).