You can apply online for TAN through the TIN NSDL portal. An application needs to be made in Form 49B. TAN is allotted by the Income Tax Department within 15 days from the date of application.
No documents are required for making an application except Basic Information of a taxpayer. However, the signed acknowledgment which is generated after filling up the form should be forwarded to NSDL e-Gov.
Read More: Step-by-step process to apply for TAN