This guide requires a user with Form Manager or Restricted Editor role with Individual form access. If you don't have access to Forms, you will not be able to make the necessary changes.
There are two ways to make a form element required:
When building or editing the element
At the bottom of the element edit popup, you will see a validation section. If you check required you will have the ability to add a Warning message, then save the element.
On the form Build page
On the build page you will see your list of elements, and there is a column called Required, if you check the box and save the form that element will be required. Even though this element is required, it will only use a default Error message "Please fill out this field". If you want something more meaningful you will need to follow option 1.