Wagner, who allegedly masterminded the killing of the Johannesburg teenager, told the court in Alberton they decided to leave one of their friends out as only five people were required to form the five-pointed star needed for the ritual.
Wagner said the friend accompanied them to a hill, in Linmeyer, where Theologo and a friend of hers were doused with petrol and set alight in an alleged satanic ritual in October 2011. She died from her burns a week later. Her 14-year-old friend survived.
"We dressed in black to prepare for the ceremony," Wagner told the court.
Theologo did not know of their plans, but thought they were going to the hill to drink alcohol.
Wagner had suggested that they hold the ritual there.
"I suggested the mountain 'cause I had been there before... .We couldn't have done it at the flat [his flat] because we needed an open area. We thought of going to the park, but we saw it was too open," Wagner said.
He told the court he packed candles, a knife, and Bible in a bag and they all headed to the koppie.
They smoked dagga on their way there and ran away when they came across a police van.
Wagner said Theologo had asked along the way why everyone was dressed in black.
"She said she too wanted to be dressed in black. Someone gave her a black jacket to put over her pink top," he said.
Wagner, Harvey Isha, Robin Harwood, and Courtney Daniels are charged with murdering Theologo and attempting to murder her friend, and with assault with intent to cause grievous bodily harm. All have pleaded not guilty.
Two other accused, Jeremy King and Lester Moody, confessed to the crime and were each sentenced to 17 years' imprisonment, five of which were suspended.
The trial continues.