First of all you need to figure out which ITR form you should use while preparing the returns of your father. This image should be a good reference for you to choose.
You can download these forms from here.
The next step is to fill the return. Here, you need to report all the income your father has earned during the year. You should also fill in the details of Advance taxes paid and TDS in the return.
Now, your father’s taxable income would be calculated, and the tax liability is determined. If your father has paid more in taxes than this amount, he would receive a refund of the excess amount of taxes paid in his bank account.
Edit: If you are filing after 31 December 2018, you will have to pay a fee of Rs 10,000. Also to note that the time limit for filing a return late for FY 2017-18 expires on 31 March 2019.
Since this period has already passed for you, you can file your returns as per the provisions of section 192. You need to file your returns with your Assessing Officer personally, with a hard copy of the return. This type of return can be submitted for a period of up to six years, but you should have a genuine reason for not filing the return on time.
If you want any assistance in the above matter, you can contact me here