Hi @Nithin,
You might want to see this earlier thread started today by someone with the same question:
I'd like to point out that we don't intend for people to use Let's Encrypt this way because Let's Encrypt certificates are valid for only 90 days and some of the people involved in the project hope to be able to make the validity period even shorter in the future. Let's Encrypt is intended to be used with software integration methods that include automated certificate issuance and renewal requests. Most of these don't require you to create your own CSR at all (although some of them can work with a CSR for use cases like certificates for embedded devices that can't run a Let's Encrypt client application directly).
We would like to encourage people to find ways of using Let's Encrypt that will be automated rather than manual.