Collard said he had been alone all day, playing computer games in his room at his parents' house in Enfield, north London, before going to repair a garden gate in the alley.
Collard, who admits manslaughter but denies murdering Katrina on 28 May this year, broke down frequently as he recalled what happened.
He said he spotted another schoolgirl walking down the alley as he mended the gate and had an urge to touch her body. He went to his room to get an imitation firearm. 'I wanted to scare a girl with it - I wanted to touch a woman's body,' Collard told the court. 'I was going to satisfy my sexual urge by touching a girl and masturbating.'
As another girl approached the gate in the alley he went up behind her. 'I held the gun to her head. I pulled her back from behind against the wall in front of my garden gate. She was panicking . . . I told her to keep quiet. I started to touch her on the thighs and her breasts. That is when she started to scream.'
Collard pulled her into his garden shed. She was still struggling and asking what he was doing. He told the court he indecently assaulted her and the girl made for the door screaming. 'All I wanted to do was stop her screaming. I . . . put my arm round her throat and pulled her back. My hand was over her mouth. She went down to her knees. She started struggling and screaming when I had my arm round her throat.
'I told her to be quiet. She went down and went forward. She stopped screaming and moving. I was just pleased she had stopped screaming. I did not realise she was dead then.'
Katrina was found the following day during a house-to- house search. A plastic bag had been taped over her head and her hands taped in front of her.
Collard, who has no previous convictions, said that he had had sexual intercourse with a girl only once. He had had a girlfriend for several months but was not going out with anyone at the time.
He admitted he had taken women's underwear from washing lines and agreed he had women's underwear in his room at the time of the killing.
The trial was adjourned until today.