You can learn more about setting up the Marketing Library integration with Redtail here. In this post I just want to cover how you would go about merging a Marketing Library document you've pushed to Redtail with multiple contacts at one time.
From your Advanced Search page, after running a search that will include your desired contacts, place checks to the left of the names of those contacts for whom you want to merge the Marketing Library piece:
In the example above, I've run a search that yielded 99 records (only partial results are displayed in the screenshot) — I then selected six of those records to whom I want to merge the document.
My next step would be to click Options in the Result Options widget to the right of my search results and then to select Create Marketing Library Document from that menu:
After clicking Create Marketing Library Document, I'll then see the Create Marketing Library Document dialog, where I can choose from the Marketing Library documents I've pushed to Redtail to merge one of them with these six contact records:
The Documents available here are those that your database users have pushed from Marketing Library to Redtail — you can learn more about how to push those documents here.
To select a Document, just click the arrows in the Select Document box, click your desired Document and click Create Marketing Library Document. Your download will then begin, merging the document for each of the six records with included merge fields (if any).