Step 1: You can create a new popup from Global Settings - Popups.
Step 2: Select Add New Pop, proceed to give a name for your popup and select Create New Popup.
Step 3: On selecting Create New Popup. You will be taken inside the Popup Builder editor where you can design your popup.
Step 4: On the left side, a set of prebuilt popup templates will be displayed, you can select an preset or continue with an bare popup structure.
Step 5: You can save and close the Popup Builder by selecting the Close Icon present on the top right.
Step 6: Now it time to link your popup to a button. You can add a new button or select an existing one and set the Click Action to be Popup, then select the popup.
Step 7: That's it. Now that the popup is linked to a button, clicking on the button in preview or published page will display your popup.