In Microsoft Office Word 2007, click Start Mail Merge in the Start Mail Merge group on the Mailings tab, and then click Step by Step by Mail Merge Wizard. Under Select document type, click Letters. The active document becomes the main document.
On the following screen, Word will ask what document you want to use for mail merge. Click the Use the current document option to use your current letter for the task. If your letter is in another Word file, click the Start from existing document to open that file. Then click Next: Select recipients at the bottom.