Sarah provided you with some good questions to ask yourself before adding this part to your business. I do a lot of Skype training as well as addition to my other training. But Skype only requires you to have available time and be able to teach your clients the exercises and routines in an effective way since you are not in front of them (sometimes it can be challenging).
One way to do online training would be for you to videotape as many exercises as you can think of and add them to your site. Then when you are giving people exercises and routines to do as part of their workouts, they would have a guide to refer to (if needed). You can either come up with programs that last a few weeks in duration or sell them one workout/day at a time. This is up to you. You will need to be able to answer any questions these clients would have about their workouts, so email would be an essential part of this type of method I believe.
Like I mentioned earlier, Skype training is my preferred method of training people who live away from me, because this is the closest it gets to a face-to-face training even though we can be miles away and it’s easier for me to keep an eye on them in case they need any type of modifications or motivation or even accountability.
I hope this helps. Good luck!