The homeless woman accused of stabbing a California mom as she changed her baby inside a Macy’s Herald Square bathroom told The Post “voices” in her head ordered her to “kill.”

“It said that I had to go to Macy’s and kill someone,” Kerri Aherne said during an exclusive jailhouse interview Saturday.

Aherne – who still had blood under her fingernails Saturday as she sat in the visitor’s center of the Rose M. Singer Center on Rikers Island, where she is being held on an attempted murder charge – described in chilling detail how the attack unfolded.

The 43-year-old former home health aid from Massachusetts took a green cab from her transitional housing unit at the Manhattan Psychiatric Center to the famed department store, she said.

She bought a smooth, foot-long kitchen knife on the 8th

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