Hello @psidello,
As of the moment, there isn't an integrated way to create a dummy company in QuickBooks Online. This is because creating a new company also requires an active subscription after the trial ends.
Instead of creating a dummy company, you can use our test drive in creating tasks for practice before doing it on your actual company. You can access it directly with this article: QuickBooks Online Test Drive.
Additionally, I've also included the following references below for a compilation of articles and tutorial videos to help you with daily recording entry:
If you have any other questions, please let me know by leaving any comments below. I'll be here to lend a hand.