Step 1: Add the appropriate fields to your form, including a field with options (like a Radio Button field) and a field that you want to show or hide based on the choice made in the previous field.
Step 2: Select the field you want to show or hide, and click the Logic button in the top left corner of the builder to dive into your form's Conditional Logic settings.
Step 3: Select when and under what circumstances you would like the field or section to show on the form. After you're done, the field will show up with a distinguished background in the builder.
To learn more about setup, please read this help article.