Angela Wrightson had more than 100 injuries on her body following her five-hour ordeal at the hands of the girls, who were 13 and 14 at the time.
During the attack, the pair took photos of themselves – and posted a picture on social media which was taken in a police van after the incident.
The girls, who are now both 15, wept in the dock after they were convicted at Leeds Crown Court.
Ms Wrightson was found dead in her blood-spattered living room in Hartlepool, County Durham, in December 2014. Police said that of her 100 injuries, 25 were made by weapons which included a coffee table, a computer printer, a wooden stick laced with screws, a television set, a shovel, ornaments, a picture frame and a kettle. Miss Wrightson was assaulted in 12 separate locations around the room.
While at the house, the younger girl made a phone call over Facebook to a friend who heard her say: “Smash her head in. Bray her. F****** kill her” as another laughed in the background.
The court heard that the defendants left the house for “time out” at around 11pm, during which time they went to see a friend, covered in blood. The pair returned to the scene at around 2am and stayed for a further two hours before calling the police to take them home. Officers who collected the girls described them as laughing, joking and in “high spirits”.
The trial heard that the girls had visited Miss Wrightson on a number of occasions as she would buy them alcohol and cigarettes.
Both had been drinking before they arrived at the house, and the older girl told the court she had taken prescription drugs earlier in the day.
Miss Wrightson’s family said yesterday she “had touched the lives of so many people”.
In a statement, they said: “Angie was attacked and brutally murdered in her own home, a place where we all have the right to feel safe.
“No sentence, regardless of its severity, will ever bring Angie back. ”
The teenagers are due to be sentenced tomorrow.