Tax paid by assessee after the 31stMarch of the previous year but before filing of return of income is called self- assessment tax. Self-assessment tax can be deposited through challan ITNS 280 by online mode or by hard copy of challan via the designated bank.
For example, If Previous year is 2018-19 (AY 2019-20) and the tax return is filed on 31st July,2019 then tax paid between the period from 1st April 2019 to 31st July 2019
Self-Assessment Tax or Advance Tax is to be deposited to the credit of Government by using the challan prescribed in this behalf, i.e., ITNS280.The Challan can be downloaded from https://www.incometaxindia.gov.in/forms/107010000000345598.pdf
Tax can be paid in the designated banks through two modes, viz., physical mode, i.e., cash/cheque or e-payment mode by using debit card or internet banking. In case you are not able to download challan after the e-payment, then challan details will be updated in your Form 26AS after few days.