After Adding Sezzle as a Payment Option Within Shopify, the next step in the integration process is to add Sezzle to your product and cart pages and install the Sezzle checkout button to drive conversions and increase your sales volume. See step-by-step instructions here and the video below for instructions:
Also, see more Shopify widget resources below:
How do I add widgets to my product and card pages? (App Blocks method)