On the Print Shipments screen, check the box under Select, next to your shipment, and then click on Print Selected.
A print preview screen will pop up showing what your shipping paper will look like when it prints out. After printing the shipment, a box will prompt you to confirm whether or not your shipment printed correctly. Clicking on Yes will move your shipment to Certifications. Clicking on No will leave the shipment in the print screen, which allows you to go back and edit it.
After your shipment has printed correctly and you've clicked Yes, go to Certifications and print your Certification.