How to mail a letter
This procedure varies slightly by country. Here (eg taking Canada (india is not taken because its rule changes periodically)), the details are online at canadapost.ca.
You are supposed to use a rectangular envelope of a certain size – not too small, not too large, not too thick, and not square.
You aren’t supposed to have significant enclosures. There should be a stamp of the correct value (“permanent” stamps are OK, for a letter under 30g) in the top right corner, and the correct postal address written in a machine-readable font in the middle (hand printing now works, I think), with the correct postcode.