How do I create an accountants copy from QBs Online for our tax accountant?

You don't, there is no copy you can make that would be any good.  In theory you could download a trial version of desktop, export your online file to desktop, create an accountant copy, import the accountant change file and then create a new online company with that data, which will lose all the data you work on online while the accountant does her magic.  Instead, you invite your tax accountant as one of your allotted accountant users. They get unfettered access to your file while you work