If your contact list file is not already housed in a spreadsheet, you will need to migrate the data from your current list into a spreadsheet program such as Microsoft Excel or Google Sheets.
If your contact list file is already in a spreadsheet, saving it as a .csv is easy!
Click "File" and select "Save As." When the save menu pops up, change the File Format to "Comma Separated Values (.csv)" and save it on your computer.
Instructions with images: How To: Export a Contact List from Excel
Click "File" and select "Download As," then select "Comma-Separated Values (.csv, current sheet)" and save it on your computer.
Instructions with images: How To: Export a Contact List from Google Apps
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