Format your addresses in Excel including column headers at the top of each column of data (i.e. first name, last name, company, address, etc) and ensure zip codes are formatted correctly so if they start with a 0, the 0 is not dropped off
Open a blank Word document
Click on the Mailings tab > Start Mail Merge > Labels
Select your label vendor (i.e. Avery US Letter) then product number (i.e. 5160 address labels)
Click on OK. You should now see an empty table in your document.
From the Mailings tab, click on Select Recipients and select "Use an existing list"
Select your Excel file of addresses and click on OK. Your document should now be blank in the top left cell and display «Next Record» in the rest. Click in the top left of blank cell if your cursor isn't already there.
From the Mailings tab, click on the arrow under Insert Merge Field and add contents exactly how you want it to appear on your labels, adding spaces between first and last name and carriage returns to align the address and any other information correctly - you can always update this if it doesn't display correctly on your first try
You can now format the contents, such as making the customer or company name bold by highlighting those fields and formatting the text with the options on the Home tab.
Once you've formatted the fields in the first cell, go back to the Mailings tab and click on Update Labels. The contents may look a little strange at first, but you can see how they will appear by clicking on preview results. Click on preview results again to go back into edit mode. If you need to make adjustments, make them on that first label again, then click on update labels again.
When you've perfected your labels, I like to view them one more time, so I click on Finish & Merge > Edit Individual Documents, select all and click OK. This will open the finished labels in a new document, enabling me to view every label to ensure text isn't wrapping to the point where an address won't get printed correctly. If more edits need to be made, just go back to the original Word doc, make your updates and repeat.