Usually, the admission process starts in the month of March/April when the colleges announce the notification and application forms, both online and offline.
Few universities take admission in M.Sc. Mathematics courses through merit basis as well as through entrance tests. Delhi University is one such university which has reserved 50% of its seats for merit basis admissions.
Colleges like Loyola College, Chennai, IIT Kanpur, IIT Delhi conduct entrance tests for taking admissions into this course. Students selected through entrance test would further have to go through a personal interview before their final selection.
Various Universities provides lateral entry into the course but keep eligibility criterion of having a graduation degree with mathematics as the main subject. One can apply to the universities and show them the documents, pay the fees as required by the college. Some of the institutes providing lateral entry are IIT Kanpur and IIT Delhi.