Step1: Click on the third last tool from the toolbar and select Buy Now Button
Step2: The Buy Now Button will appear on the block and you will get a toolbar at the right corner of your page. Click on settings.
Step3: From here, you can label your button, select product, select the payment gateway and then click on 'Save'
Step 4: Click 'save now' and the payment gateway will be connected to your checkout button.