Webinars are just seminars done virtually.
Create a seminar as described in "How to create a conference, seminar".
To make it a virtual event, from the Design Event Campaign tab of your event, go to the Landing Page tab -> go to the Overview section -> Virtual Event header. Check the box that says, ‘This is a Virtual Event’. This will set the campaign as a virtual seminar i.e., a webinar.
Once it is set as a webinar, the Venues tab will not be visible anymore and you will not need to fill in the physical venue details.
There you go, your webinar is set to run!