Shop around. There are a number of services that offer this form of educational interface. You can compare them by criteria such as price, length of the course, expectations of the course or certificate offered at the end. There may also be specific webinars that pertain to your line of work.
Register for the course. This will typically require you to submit payment in advance for the course. Once registration and payment have been received, you may expect the service to alert you by email as to the next step.
Attend the classes. The service will inform you when to attend class through logging into your computer. Before the first class, make sure you have all the necessary equipment for the interactive experience such as video camera, microphone, headset or all of the above. Test them to make sure they work so you are not dysfunctional for the first introductory session.
Practice vocalization. One of the benefits of a webinar is that you are usually conversing with native speakers, so use that edge to refine your pronunciation and get feedback. Keep practicing and do not be afraid to ask questions. That is exactly what the instructor is there for. Do not be discouraged if your pronunciation is not where you want it to be at first.
Print out any materials. The course/instructor may provide you with supplemental documents or resources to augment your learning. They will probably be in a printer-friendly format so you can produce them right at home. Thoroughly review them and have them handy during class time for easy reference.
Do the homework. Just because you are not in a physical classroom does not mean you will not be held accountable for doing the out-of-class work. You want to be in sync with the instructor and your fellow students. Homework is introduced so that you get the most out of the course and, ultimately, the most for your money as well.