Gilgo Beach murder suspect Rex Heuermann kept meticulous notes on his kills — including reminders for future murders like “hit harder” and to get more sleep to have more “play time,” prosecutors revealed Thursday — as he was charged with two more slayings.

Heuermann, 60, who is accused of killing multiple women and leaving their remains scattered along Long Island’s Ocean Parkway, appeared in court Thursday morning and pleaded not guilty to second-degree murder charges for the 1993 death of Sandra Costilla and the 2003 killing of Jessica Taylor.

He nodded along as Suffolk County prosecutors read aloud from an alleged “planning document” recovered from a computer in his Massapequa Park home that they say served as a “blueprint” for his twisted crimes and showed his “obvious” intention to kill the women.

The married father of two — who was first arrested last year in the murders of four young women known “Gilgo Four” — was linked to both Costilla and Taylor through DNA from male and female hairs found on their remains, prosecutors said in a court filing.

Prosecutors’ case’ so far has mostly relied on evidence that showed him in the same location as burner phones used to contact some of the victims and taunt their families, as well as DNA found on several victims that was linked to him.

But in March of this year, investigators discovered a damning trove of “planning documents” that Heuermann allegedly used to “methodically blueprint” his kills, the new filing alleged.

The shocking evidence was found on a hard drive taken from Heuermann’s basement during a search in the wake of his arrest in July 2023, prosecutors said.

“The task force believes that this is a planning document that was used by Heuermann to plot out his kills with excruciating detail,” Suffolk County District Attorney Ray Tierney told reporters after the arraignment Thursday.

“His motivations, meticulous planning, and clear intent was obvious. His intent was nothing short but to murder these victims.”

Heuermann’s lists included notes on potential or past dump sites, as well as supplies that he used to try and evade detection as he allegedly targeted, tortured, and killed the young women.

During his court appearance, Heuermann appeared to nod along when the prosecution read out an excerpt from his list that suggested he should “‘hit [the victim] harder next time.’”

One list marked “POST EVENT” included reminders to “change tires,” “burn gloves,” “dispose of pics [of victims],” and “have story set,” while another titled “PROBLEMS” noted the potential pitfalls of DNA evidence or fingerprints.

The list titled “BODY PREP” included notes to wash the “inside and all cavities” of his deceased victim and to remove their “head and hands” and “marks from torture.”

Another list detailing post-crime cleanup referred to strategies for hanging drop cloths from the ceiling and concerns about how sound travels – leading investigators to believe that the torture “likely” took place inside Heuermann’s home.

Heuermann even noted that getting more sleep was important in order to have “more play time” – the sick euphemism he supposedly used to refer to torturing and killing the victims.

Authorities also found Heuermann’s “significant collection” of torture porn, including images of breast mutilation and vaginal torture.

The architect had also compiled what appeared to be notes from a first edition paperback of “Mindhunter” by retired FBI agent John Douglas, the prosecution noted.

Heuermann also had a copy of Douglas’ “The Cases That Haunt Us,” which included analysis on victimology and other evidence in some of the United States’ most infamous crimes. Douglas did not reply to The Post’s request for a comment.

The content on the hard drive was a “significant” factor in investigators’ decision to search Heuermann’s home again last month and to deploy K9 units at the sites where Costilla and Taylor’s remains were found, Tierney said at Thursday’s presser.

The body of Sandra Costilla, 28, was discovered by a pair of hunters in a wooded part of Southampton on Nov. 20, 1993.

Her body had been arranged with her arms out above her head and her legs spread open, court documents filed Thursday.

Her shirt had been pulled up, and she had 25 total sharp force injuries to her face, torso, breasts, vaginal area, and left thigh.

The gruesome injuries are believed to have been mostly inflicted postmortem, Tierney said.

Shortly after her remains were found, forensic examiners found unidentified female and male hairs on Costilla’s remains, the document stated.

In 2014, the hair evidence was used to rule out suspect John Bittrolff – a convicted murderer who was believed to have been tied to Costilla’s killing, the new charging documents revealed.

In February and March 2024, the male and female hairs found on the body were found to match Heuerrmann and the unnamed woman he was living with at around the time of Costilla’s murder.

The woman – who is referred to as Witness 3 in the bail application – moved out of Heuermann’s Massapequa Park home in September 1993, about two months before Costilla’s disappearance and death.

Heuermann’s mother had also left the home shortly before Witness 3 moved out, the bail application stated – suggesting that Heuermann had free reign to kill Costilla and transport her body out east.

Unlike Heuermann’s other alleged victims, there was no record of Costilla having been a sex worker, early reports indicated. The Suffolk County DA’s office did not return The Post’s request for a comment on the matter.

Jessica Taylor was 20 years old when she disappeared on July 21, 2003.

She was last seen in person in New York City on July 19, and was reported missing when she failed to return home to Poughkeepsie for her mother’s birthday, prosecutors said.

Her torso and legs were found by a dog walker in a wooded area of Manorville on July 26, 2003.

She had been decapitated and both of her arms were severed from her body below the elbows. A tattoo on her torso had also been “severely obliterated by a sharp object,” the filing stated.

About 13 hours before the dog walker found Taylor’s partial remains, another witness reportedly saw a dark-colored Chevrolet pickup truck backed into the wooded area where the mutilated torso was discovered, the application said.

Internet search data revealed that Heuermann shopped for a new, light-colored truck in the days after Taylor’s remains were found – despite the fact that his own dark green Chevy was only about four months old.

Taylor’s skull, hands and forearms were found on March 29, 2011, along Ocean Parkway, just east of Gilgo Beach – less than a mile from the bodies of the “Gilgo Four.”

Authorities now believe that the extensive mutilation “were acts perpetrated by Rex A. Heuermann to…inhibit the identification of the victim,” court filings said.

The initial examination of Taylor’s remains in 2003 brought up a male hair on the surgical drape found underneath the victim. Earlier this year, a DNA sample from that hair was found to be a match to the buccal swab taken from Heuermann.

Taylor was known to have earned a living as a sex worker in midtown Manhattan, not far from where Heuermann’s office was at the time.

An interview with one of Heuermann’s daughter’s former classmates also revealed that his wife, Asa Ellerup, took their two children on vacation to Vermont between July 20-27, 2003.

The trip was subsequently confirmed via phone records and electronic data.

Heuermann’s family was also notably out of town during the dates he was accused of targeting and killing the Gilgo Four between 2007 and 2010.

Taylor’s mother, Eilzabeth Baczkiel, attended the Thursday morning press conference with attorney Gloria Allred.

“Jessica, my darling daughter, you will never be forgotten. You will always be missed,” Backziel said in a statement read by Allred.

Taylor was remembered by her family as “loving, compassionate, and so funny.”

Heuermann’s defense attorney, Michael Brown, told The Post that his client was “startled” and “adamant about his innocence” in light of the new charges.

When asked about Heuermann’s behavior in the courtroom Brown said he was mostly focused on processing what was happening.

“He wasn’t looking at much. You’re up there in handcuffs before a judge. He wasn’t really scrutinizing it the way you or I would,” the lawyer insisted.

Investigators are still going through data from a massive trove of devices linked to Heuermann, including 15 different types of cameras, 27 computers, and 46 cell phones, Tierney said after the arraignment.

Thursday’s proceedings bring the total number of murders Heuermann stands accused of to six – and prosecutors indicated even more charges could be on the horizon.

Prosecutors believe that one of the sections on the planning list could be tied to another victim, Valerie Mack, a 23-year-old escort whose partial remains were found near Taylor’s body.

While Heuermann has not been charged in Mack’s death, prosecutors said it’s “fair to say” that he’s a suspect in the case.

“We are going to continue to investigate. We’ll talk about whatever other investigation we’re undergoing when and if we bring those charges,” Tierney said.