No worries - if any of what I've said doesn't make sense, just say.
One further thought - if you are doing it spearately on your own as above, you'll probably want to warm up beforehand - in which case, after your warm up you'll have to exit Zwift and then re-start it and follow the above instructions. You'll have time to ensure your bike is changed to the TT bike etc before starting once you appear by the side of the road, as the Strava segment we'll be using won't start until you go under the start/finish banner just up teh road, so no need for a mad panic to ride off as soon as you appear on course.