You can just copy paste the link provided below for NIOS admission procedure. A very well composed flowchart is provided which will make it easier for you to understand:

https://sdmis.nios.ac.in/static/dist/images/pdf/stream-1/block-1-nios-admission.pdf

As a eligibility criteria you have to pass your 10th examination.

If you want to give your NIOS 10+2 examination in April/May 2022 then you have to get admitted within 16th March - 31 July,2021.

For the detailed dates for admission you may visit the link below:

https://sdmis.nios.ac.in/

I hope my answer helps. All the very best for your future endeavors!