Hi Rose,
The process of filling the AIIMS Nursing 2020 application form has three stages - Basic Registration, Generation of Code for Final Registration and Final Registration (Payment of fee and City Choice). The basic registration is known as PAAR which is the "Prospective applicants advanced registration". Only those, Completing the PAAR can move to the second stage. PAAR allows candidates to register only once and use the details to apply in future also.
To fill the form following are steps:-

• To fill personal details
• Uploading images
• Printing the form

After this step 2 which is code generation is performed. And then step 3 final registration which includes city selection and payment of fees is performed.
If you want detailed procedure and steps please check this link:-
https://medicine.careers360.com/exams/aiims-nursing