To edit a form that's already on your page (e.g. if you start with one of our high-converting templates) just double click on the form fields to open the "Design Your Form" dialog.
To insert a form just drag and drop the Form widget from the panel on the left. Note: only 1 form is allowed per landing page.
From the "Design Your Form" dialog click from the Custom and Pre-defined Fields on the left to add them to your form. Click your chosen fields in the middle to select them. Once selected, move them up and down or delete them using the black arrow buttons or red delete button and adjust their available options on the right (included making them required).
Single click the form fields and you'll see the Form Properties on the right of the Page Builder: