Dear Aspirant, the preparatory course admission process in IIT has the following conditions :-

1. The seats reserved for the respective category should be vacant.
2. Candidate should be satisfying the minimum norms.
3. Candidate should not have been admitted for a preparatory course in IITs earlier.

The process of filling choices in preparatory courses is done through the online portal only. I hope you are clear now. If you have another queries feel free to ask. All The Best!!!