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For example, if you wanted to print a set of mailing labels, you would construct a source document containing the addresses of the people you wish to print labels for and a main document that contains information on where to put each person's name, address, city, state, and zip code on the label. The main document would also contain information on how many labels are on a page, the size of each label, the size of the sheet of paper the labels are attached to, and what kind of printer you will use to print the labels. Running a mail merge with the two files results in a set of labels, one for each entry in the source document, with each label formatted according to the information in the main document.

Instructions

See Office - Quick Reference for Mail Merges.

Microsoft has instructions for running several types of mail merges on their web site. 

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If you are performing a merge to e-mail and want to send from a shared/departmental mailbox, you will need to set up an Outlook profile with just the shared mailbox first. In step 11 13 there, select to be prompted for which profile to use when launching Outlook.