Ok - Let's go through the wizard - but before you do make sure that your list in Excel has a column header e.g. Part_Number - this will be used as a reference during the merge. Here's what I do and see ....
On a blank Word document I click on Start Mail Merge and click on Step by Step Wizard
In the side bar that appears I click on Labels and then continue
On the next side bar screen I click on Label Options and this will bring up a pop-up where you define the dimensions of your label sheet. If you are using a known brand of labels it might be in the label list - you'll need to find the particular code for the sheet that you have and Word will organise it for you. If you are not using a common brand of sheet you can define it yourself by clicking on New Label and entering the appropriate dimensions
Once that's done and you click next you should see the label outlines on the document
Click on "use an existing list" and then Browse to call up your excel spreadsheet. Select the relevant sheet from the list of sheets in the excel file and it will show you each of your labels with a check mark next to it. Note the headers in this window - you should see "Part_Number" as the header. If you see all your part numbers click on Okay
This will put <<Next Record>> in each label area.
Click on next-Arrange your labels
Click on "More Items" and it should show you "Part_Number" click on Insert and close. Then, in the same side bar click on Update all Labels. This will populate all label areas with "Part_Number"
Click on Preview and you should see the contents of your excel populate the labels.
(I used a list of tv shows I copied and pasted into Excel for demo purposes only so ignore that)
If you do click on Complete the merge and then print.
If you chose the right Label dimensions the printer should place each part number correctly.
Give this a try and if you get hung up on any of the steps post back.