Hilchos Maasei Hakarbanos 6:22

Leviticus 1:17 says that the kohein “shall split (the bird) by its wings” This is done manually, not with a knife. The halves of the body need not be completely separated from one another, as the verse continues, “he need not separate it,” but if he did separate it, it is valid. He dips it in salt and throws it on the fire. If he didn’t remove the crop, the feathers or the innards, or if he didn’t dip it in salt, it remains valid. If a deviation from procedure occurs after the squeezing of the blood, the offering remains fit.

Hilchos Maasei Hakarbanos 6:23

Melikah (nipping the head) is performed as follows: The kohein cuts downward with his thumbnail at the nape of the neck; if he wants to move his thumbnail back and forth, he may do so, and if he wishes to press and move downward with his thumbnail, he may do so. If the parts necessary to cut for ritual slaughter move from their place, it’s not a problem. He must cut the spine and the neck without cutting most of the flesh; if he cuts most of the flesh, it’s considered dead before he reaches the organs that must be cut for ritual slaughter (the windpipe and the esophagus). In the case of a burnt offering, he must cut both of these things in his downward motion. If one performs melikah with a knife or from the side, it’s not melikah. Rather, it’s like strangling or beheading. The whole nape of the neck is a valid spot to perform melikah.