how do I convert excel sheet into mailing labels

Make certain in your spreadsheet that there are no blank lines, duplicates, and all data fields are correctly populated. In the Contacts application, make a new named group, and select it. From the File ▸ Import… pick your Excel/Numbers CSV spreadsheet, and click Open. Each address line in your spreadsheet will be a new contact within your selected group.

After import, reviewe your new group contacts to ensure accuracy and completion. With the group selected, File ▸ Print. In the Print panel, choose the following:

• Printer: Where you are going to print the labels
• Contacts
• Style: Mailing Labels
• Layout tab: Avery Standard and label number (e.g. 5161).
• Label tab: Addresses: You can print all in the group, or if members of the group have different types of addresses, you may wish to pick and choose by editing a distribution list in the Addresses selection.

Take advantage of Contacts Help, and the ? on the print panel. The following documents may be useful:

• Contacts (Yosemite): Print labels, envelopes, and lists
• Contacts (Yosemite): Import Contacts
• Contacts (Mavericks): Print contact information
• Contacts (Mavericks): Export and import contacts