You can fill the Open Class 12 (Senior Secondary) form online through the National Institute of Open Schooling (NIOS) admission portal. First, visit the NIOS admission page and select Senior Secondary Course (Class 12). Then register by entering your basic details and fill in the application form with your personal information, Class 10 academic details, preferred subjects, and study centre. After that, upload the required documents such as your photograph, signature, ID proof, and Class 10 marksheet. Finally, pay the admission fee online to successfully submit the form. Generally, if you have passed Class 10 and are at least 15 years old, you are eligible to apply for NIOS Class 12 admission.