I've been keeping Pettigrew since a few months ago. I got it when the Weasleys were distracted from Ginny's disappearance.
One day during the summer holiday, I was bored. So, I just take him to the family altar and stun him. After I force-shift him back to his human form, I slit his throat and let him bleed to death after a few incantations.
After he was dead, I burned his body and banished the ashes.