Yesware's Mail Merge uses CSV files to populate your emails with data. You can create a CSV file in Excel.
1. Create a new Excel spreadsheet
2. Each piece of data that you'll want to merge into your email will have to be in its own column on the spreadsheet. Whatever you title each column will determine the name of the field when creating your Mail Merge. NOTE: you must use "Email" as your first column
3. In Excel, choose File > Save As. Change the format to "Comma Separated Values (.csv)". After pressing Save, Excel may warn you about features not working in the selected format - that is not a problem.