We present Onebody, a virtual reality system for remote posture guidance during sports or physical activity training, such as martial arts, yoga or dance, using first person perspective. The system uses skeletal tracking of the instructor and the students, rendered as virtual avatars. Using a virtual reality headset, the student can visualise the movement of the instructor's avatar, rendered in place of their own body. Onebody provides a first person perspective of the movement instruction, allowing the student to step into the instructor's body. We conducted a study to compare the performance of Onebody in terms of posture matching accuracy and user's preference, with existing techniques of delivering movement instructions, including pre-recorded video, video conferencing and third person view virtual reality. The result indicated that Onebody offers better posture accuracy in delivering movement instructions.