Explain how Dr. Shepard commited the murder?

In a stunning plot twist, Poirot reveals that it is Dr. Sheppard who is the murderer. Sheppard stabbed Ackroyd before leaving him that night, programmed the Dictaphone to go off at 9:30 and provide him with an alibi, then snuck around the side of the house, crawled into Ackroyd’s study, locked it from the inside, and planted the footprints with Ralph’s shoes in the mud. He hid Ralph so that the police would find him more suspicious. He murdered Ackroyd because it was he, Dr. Sheppard, who was Mrs. Ferrars’ blackmailer, and he knew if Ackroyd found out he would be ruined.