@Shubhi
You have to send the application for renewal from the same office that issued the original license
I am guessing in your case that is the RLO in Kolkota
In addition to a gazetted officer attested photocopy of the logbook showing 20 hours of flying you need a letter from the flight school verifying the hours
I dont think there is specific format just a formal letter addressed to the WPC office on the flight school letter head mentioning your RTR number etc and the number of hours of flown and on which specific aircraft
Since you passed Part 1 and Part 2 you are eligible for renewal of license for lifetime validity after paying a DD of 10,000 rupees
Then why renew only for 20 years and run after the WPC 20 years from now for renewal again . Get the lifetime license and forget about it for life
The late fee is calculated as 2% of the DD amount with a minimum amount of 250 rupees ( please confirm this with someone at the WPC about the minimum amount )
I believe the late fee is added to the amount for issue of license
And yes in the COP 23 application form you can mention the name of the flight school