Step-by-step instructions for Chrome, Firefox, and Safari:

Before you continue, your printer should have already been setup and listed in your Printers & Scanners in System Preferences.(Staus should show Idle)

1) Buy the label and click the “Print Label” button.

2) At the top of the popup, select the 4x6" label format.

3) You'll see a label preview if you're using a web browser that supports it, like Chrome.

In Chrome,  put your mouse over the label preview and click the little printer icon on the top right.

In Firefox, click the “Download Label” button and Firefox will ask you what action to take. We recommend opening the PDF with Apple’s Preview.

In Safari, hover your mouse over the label preview, and click the 2nd icon from the right to open the PDF in Apple’s Preview:

4) For Firefox or Safari, once you have the labels open in Apple Preview, you can go to File > Print or use the hotkeys Cmd + P to open Preview’s printing dialogue box.

5) Chrome and Apple Preview's printing dialogue boxes are arranged a little different, but they both have the same basic options.

First, make sure the printer you want to use is selected. In Chrome’s printing dialogue box this is called destination, just click Change. In Preview’s printing dialogue box the destination is called Printer.

Next make sure the Paper Size is a 4x6" option or 100mm x 150mm. If you do not see either of these options, you likely have not installed the printer drivers correctly. Follow the directions that came with your printer.

Sometimes, “Fit to page” or “Scale to Page” are helpful options.

6) That should be it! Click Print and your label should print as expected.

Reference: Mac: How to print with a 4x6" label printer