You need to visit the NIOS website nios.ac.in or the Student portal of NIOS sdmis.nios.ac.in to apply. Here is the link given below for the further instruction & read Page no. 8 & 9 to apply for NIOS.
http://nios.ac.in/media/documents/prospectus/ACADEMIC_PROSPECTUS_2018_19_FINAL.pdf
Good Luck!
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