A court heard Martin Bell was hearing voices from god and believed in witches at the time he killed Gemma Simpson with a hammer at his home in May 2000.
Bell then put Gemma in a bath, covered her with water and left here there for four days.
He then sawed her legs off in order to fit her body into a sleeping bag which he secured with chains and a padlock.
Bell, 45, then put the body into a hire car and drove it to Brimham Rocks, near Harrogate, where he buried it.
Gemma was 23 at the time she went missing from her home in Harehills, Leeds, in May 2000.
She was last seen by a friend who she told she was going to visit a friend in Huddersfield for the weekend.
But the court heard that Gemma instead met Bell, who she had known for around five years, and caught a train with him to his home in Harrogate.
After the killing, Bell avoided the attention of the police for many years, leading a law abiding life and setting up home with a woman and her children.
Simon Myerson, QC, prosecuting, described how Bell walked into a police station in July this year and confessed to the killing before showing officers where he had buried Gemma’s body.
Bell had been released from a psychiatric hospital around six weeks before carrying out the killing.
The court heard Bell had been assessed by doctors who concluded Bell had a psychotic illness similar to schizophrenia at the time of the killing.
After handing himself in Bell told police he had met Gemma at Leeds railway station on May 5 and they had travelled to his home in Harrogate and they smoked cannabis.
Mr Myerson said: “At the flat the defendant told Gemma that god wanted him to kill her and he gave her the opportunity to leave.”
He described how they argued before Bell took a hammer from the hallway and returned to the living room before hitting her repeatedly with the weapon.
Bell said Gemma was still breathing so he took a knife from the kitchen and stabbed her in the back and the head.
The prosecutor said: “He said that there was still body movement and he grabbed Gemma and put her in the bath and filled the bath and covered her with water.”
Bell said he became frightened and closed the shower curtain so he did not have to look at Gemma.
He then painted the walls and floors of the flat to cover up blood staining. Bell then hired a car Ford Focus car but the boot was too narrow to fit the body inside.
Bell told officers he sawed off the bottom of Gemma’s legs in order to make the body fit into the vehicle.
He said: “I sawed as fast as I could so I wouldn’t be sick.”
Bell said he then put the body in a sleeping bag which he secured with chains and a padlock “so she couldn’t get out.”
He then drove to Brimham Rocks and dug a hole about four feet deep before burying the body.
Bell said he visited the beauty spot four or five times over the years before her body was discovered. Officers were able to identify her her using dental records.
Mr Myerson said the state of the body was consistent with the accounts given by Bell during interviews.
The court heard Bell’s only criminal convictions were for a dishonesty offence in 1992 and a conditional discharge for common assault in 2004.
Since 2011 he had worked at a taxi driver for Atlas Cars in Scarborough.
Paul Greaney, QC, mitigating, said Bell had “delusional beliefs” at the time of the killing.
He said that Bell thought a group of men wanted to kill him and he had moved around the country because of those fears.
He said: “He thought that the supernatural world or witches was real.”
“This is a man whose way of thinking was twisted and distorted by the condition from which he suffered.”
Mr Greaney said the killing had not been premeditated and asked the court to give his client credit for confessing to the killing and helping to locate Gemma’s body.
The Recorder of Leeds, Judge Peter Collier, QC, adjourned the case until Friday when Bell will be sentenced.