When you want to merge the same main document and data source in the future, just open the main document with the usual File, Open command, and click the Merge to New Document or Merge to Printer button in the Mail Merge toolbar. That's all there is to it.
If you want to merge an existing main document with a different data source than the one you used with it previously, you can attach a new data source. Open the main document, choose Tools, Mail Merge, and click the Get Data button.
Then, if you want to create the new data source from scratch, choose Create Data Source and continue with the steps described earlier to run the merge (starting with "Creating and Saving the Data Source").
If you want to attach an existing data source, click Open Data Source, select the data source in the Open Data Source dialog box, and click the Open button. Back in the Mail Merge Helper dialog box, click the Merge button, and then click the Merge button again in the Merge dialog box to run the merge.