Two teenagers who wanted to experience murder told police it "felt right" to strangle a friend and bury her body in a shallow grave beneath her West Australian home.

"Sunday morning me and (her) woke up, and we were just talking, and for some reason we just decided to kill her," one of the girls told police in her interview.

"We just did it because we felt like it, it is hard to explain," the other girl said.

"I knew we had wanted to kill someone before.

"We knew it was wrong, but it didn't feel wrong at all, it just felt right."

One of them snuck up behind Eliza as she was reading, wrapped speaker wire twice around her throat and quickly tightened it as the other held her down, trying to press a chemical soaked cloth into her mouth.

"As our friend, we did not really want her to suffer," one told police.

"If she had died another way it probably would have bothered me ... but it just did not," one girl said.

Mr Stone called for sentences of life in prison.