Create a flow. To add logic to a form, go to the flows editor and create a flow linked to the proper stage of the form.
Add an 'If/then condition' action. Just click on the plus button of the flow and select it from the actions menu.
Set the condition. Reference the variable you want to check on in the first input and select the condition that best suits the needs of your form.
Build the ‘True/False’ branches. Add to each branch the actions that will be triggered based on the result of the previous condition.
Publish and test it. Publish the flow and click on the ‘Preview’ button of the form to check that everything is working properly.