You can find the Start Mail Merge button in Microsoft Word under the Mailings tab on the ribbon. It is located in the "Start Mail Merge" group, where you can click the button or its drop-down arrow to choose the type of document you want to create for the merge, such as letters, envelopes, or labels

. Summary:

• Open Microsoft Word
• Go to the Mailings tab on the ribbon
• Look for the Start Mail Merge button in the "Start Mail Merge" group
• Click it to select the document type and begin the mail merge process

This applies to versions like Word 2007, 2010, and later editions.