When prosecutors indicted Rex Heuermann this month on two additional murders in the Gilgo Beach serial killings on Long Island, they also described a manual he kept as a “planning document.”

Prosecutors say Mr. Heuermann created the document to “methodically blueprint” the selection, killing and disposal of victims, according to a bail application filed with the indictment.

Written in capital letters, the manual is structured as a series of reminder lists under topics like PRE-PREP, which offers banal tips about the importance of checking weather reports and looking for surveillance cameras. The PREP section is more grim, with directives to set up a holding area and “stage” with equipment for apparent sexual torture. The BODY PREP list includes reminders on how to avoid leaving evidence.

The manual is the most informative piece of narrative evidence disclosed since prosecutors filed legal papers after Mr. Heuermann’s arrest detailing investigators’ 18-month pursuit. In prosecutors’ hands, a document they say the defendant created to avoid detection may instead become damning evidence against him, even given the already disclosed DNA matches, phone records and internet activity that prosecutors say tie Mr. Heuermann to the killings.

The methods that Mr. Heuermann outlined in the document in “excruciating detail” correspond to the ways in which he carried out the six murders, said the Suffolk County district attorney, Raymond A. Tierney, adding that Mr. Heuermann’s “intent was specifically to locate these victims, to hunt them down, to bring them under his control and to kill them.”

Mr. Heuermann, a 60-year-old architect, was arrested last summer and later charged with murdering the so-called Gilgo Four. Those victims were among 10 sets of human remains found along a desolate stretch of Ocean Parkway east of Jones Beach in 2010 and 2011.