To fill in the marks for the 11th and 12th grades in the NEET application form for the AP board, you will need to follow these steps.

1. Make sure you have your marksheet or transcript for both 11th and 12th.
2. In the NEET application form, there should be fields provided to enter your marks for each subject in both 11th and 12th grades separately.
3. Double check the marks you enter to avoid any incorrect information.
4. Some applications may ask for aggregate marks or percentage. Make sure you calculate these correctly.
5. Before final submission, review all the information you have entered, including your marks for 11th and 12th grades, to ensure everything is correct.
6. Once you are satisfied with all the information provided, submit the NEET application form as per the instructions provided.