You don't, at least not through the wizard.

I create a page of the label I want specially formatted and then do the
formatting, generally through a style. Save it as a template.

If I am only typing a single label, I create a new document from that
template. I type the info in the label that I want to use. I then mark the
text as hidden and save the page for reuse. Next time, since I can see the
hidden text on my screen, I know not to use that label.

You can center the label text with Ctrl+E, which will center every line.
That's probably not what you want. If you want the text left-aligned but
visually centered, you'll probably need to compromise. Years ago I had a DOS
database program, PC-File, that would spit out address labels that were
centered as you describe, but this undoubtedly required a bit of programming
and replicating it in Word would require either a lot of manual formatting
or some tidy VBA. You can align all the addresses to a sort of average
center position, though, by adding a left indent to the label paragraph(s).