Online tutoring can be an enjoyable experience for both participants - the teacher and the student - especially if there is a mutual understanding of the informal nature that such a setting allows. Knowing from experience that Japanese students of English generally have a problem with communicative discourse, despite having a solid foundation in the form of the language, I would advise keeping it as informal as possible. Don't expect that your first few sessions will be extremely productive. Finding out what your individual students are interested in and engaging them in conversations about their interests can lead to some very successful sessions. In addition, I would advise keeping corrections to a minimum. When you have reached a point where you are clearly aware of the problem areas, you can address those problems through properly modeling the language without creating the anxiety that may result from constantly pointing out mistakes.

Good luck!