Yosef Karo

“ A non-priest may not perform the blessing even if there are other priests) .
2. Any priest, who is not disqualified, if he does not ascend the Duchan [2] , then even though he only transgresses against one positive commandment, it is as if he has trangressed against three positive commandments, if he was in the synagogue when the prayer-leader called out "Cohanim!" [3] or it they told him to ascend or to wash his hands.
3. If he has already ascended (to perform the blessing) once that day, then he does not transgress if he refuses to do so another time, even if they tell him to ascend. ”

“ They stretch out their hands and divide their fingers, by arranging them into 5 apertures: (1) and (2) - between the two pairs of fingers (on each hand) , (3) and (4) between the thumb and the index finger (on each hand) and (5) between the two thumbs. ”

“ That is the custom in these (Ashkenazi) countries.. Then (the priests) turn their faces towards the congregation. But if there is only one (priest) they do not call out ("cohanim") to him, he just turns his face (towards the congregation) of his own accord.
Paragraphs 11-15 11. When (the priests) turn their faces towards the congregation, (the priests) recite the blessing "...who has sanctified us with the holiness of Aaron and commanded us to bless His people Israel with love".
12. (The priests) lift up their hands to shoulder height with their right hands slightly higher than their left. ”