On the main Customers page, in the upper right side you will find the Export and Import buttons. Under Import is a selection called Tags.

Create a spreadsheet with two columns only:

Column A header = id

Column B header = tag

**Very Important**—Please make sure that all letters in the column headers are lower case. If you use any upper case letters the import will not work. We mention this because an upper case I in “id” catches people by surprise more often then you would believe.
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Put the customer id number into column A and in column B you put the tag you want to use.

Do not use punctuation in the tag other than a dash -

If you use commas, periods, forward slashes it will cause the tag not to function properly.

You have to save the spreadsheet as a MS-DOS Comma Separated .csv

It never hurts to test it with one client in the spreadsheet to see the upload populate properly.
Then build a larger upload.