You can only use this functionality from the native application so make sure you download Card Ladder from the Apple Store or Google Play Store.
Click the Sales tab on the bottom menu and click the search icon in the top right to bring up the search view.
Click the camera icon in the bottom right.
Take a picture of your card. Try and get the camera to focus and use good lighting.
If the card is graded you should get relevant matches of your exact card. If there's an issue with the system getting the cert off the graded card then the search results will use image recognition to attempt to find the best match visually. The results will be integrated into the Sales view so you can further filter down the results and use different sorting options.