Step1: Click on the checkout button and you will get a toolbar at the right corner of your page. 2nd tool in this toolbar states 'Settings'. Click on it.
Step2: From here, you can label your button, select product, select the payment gateway and then click on 'Save'
Step3: Click 'save now' and the payment gateway will be connected to your checkout button.
Learn via Video: https://s3-us-west-2.amazonaws.com/meetzippyandhqmeetuptokboxrecording/46351292/9ae40796-252e-49d1-810d-5c526540c607/archive.mp4