There is no demo page. But you can register and look at the 2019-20 pages without any commitment to actually use them.
In my experience HMRC tend to be forgiving. If you owe no tax then I expect there would be no penalty for failing to report sales over 49.2k. But you would probably only be forgiven one first year of mistake. But why not report? I find it a good way of annually reviewing my financial health and plans. Someday you are probably going to have to report. Why not learn about it now?
HMRC are responsive to questions put in direct messages to them on Facebook.