A completed form can be submitted online at the NSDL website or at the "Tax Information Network Facilitation Center" (TIN-FC).
TAN is required to be quoted in all TDS/TCS returns, all TDS/TCS payment challans and all TDS/TCS certificates to be issued.
However, where the application is online, the acknowledgment (a PDF file) which is generated after filling up the form must be forwarded to NSDL.
When NSDL receives the TAN application along with said documents (either through TIN FC / Online), the details are verified and then sent to the Income Tax Department.
Once approved, the Department allocates a unique number, and notifies the applicant through NSDL.