Refer to the following to add custom form:
Navigate to: Settings -> Custom Form
Click on the +Add button (given in the page action bar)
The newly opened page is divide into sections:
Configure
Enter the Title of the form
Select the Type of the form
Enter the Description and Introduction, click on Add button to add the custom form, now click on Manage Fields to manage fields of the form
Manage Fields
Under the section of Add Field, enter the name of the field, mark the check box to make the field mandatory
Select the Type of the field
Enter the description, enter the options (press enter after each option), click on +Add button to add the fields
Option of Preview of fields is also available towards the right side of the page