Form 15CA has to be filled online on the website of the Income Tax Department. The printout generated from this website has to then be signed by the remitter and submitted along with the Form 15CB. The detailed process of filling up the form is also available on the Income Tax Department website.
Steps to be followed to file Form 15CA online:
https://www.incometax.gov.in/iec/foportal/help/statutory-forms/popular-forms/form-15ca-um