To enable this feature in the form, go to:
Your form's configuration
Click on Add at the bottom of the list of fields
Select Signature
Select the attachment field you'd like to use to store signatures\
Alternatively, you can also navigate directly to:
Attachment field
Upload settings section
Select Hand Signature from Field Mode.
When the field is set to "Hand Signature" mode, it will automatically be set to only accept one file (the signature), and to use the "Contain" Thumbnail mode. These options can, of course, be adjusted to your preference.
If you want the signature to be mandatory, you must set the attachment field as Required.