Online peer feedback has been widely applied to teaching L2 writing because of its potential benefits. Accordingly, training is one of the prominent factors contributing to the effectiveness of the activity, especially for inexperienced learners. Acknowledging this significant issue, the current study aims to develop specific online peer feedback training for inexperienced learners based on their performance naturally during the online peer feedback process, followed by an assessment of its effectiveness. Using the qualitative method design that draws upon the sources of data, including students' online written comments (Google Docs), oral feedback (Zoom recorded), observations, their drafts, and their reflections and responses through semi-structured interviews, this study presents an in-depth analysis of students' performance toward peer feedback (without training), how the training is developed, and the impacts of training on the students' practice. Finally, the study discusses pedagogical implications regarding the instructions and training for online peer feedback in teaching L2 writing, which can be applicably adapted for similar EFL or ESL contexts and learner backgrounds.