Get the embed code for the buy button by going to the plans table and clicking on the actions button for the desired plan.
Choose the billing option and label you want to generate for the button then Select all and copy the embed code.
Navigate to the pages section in your Shopify admin. You can either add the buy button to an existing page or create a new page. Once on the desired page click the two arrows in the top right of the content section and paste the copied code.