Free ssl certificate

Hi I have a Private key and CSR where do O use them to get a free certificate

Sorry I got lost I want to know how to use my Private Ket and CSR to get a certificate

I have read this before and find it all confusing.
My hosting provider" Infinityfree" supplied the Private key and CSR and they say I should use these in
“letsencrypt” to obtain my certificate. How do I do that.

You may want to consider switching to a hosting provider that provides integrated support for Let's Encrypt--there are lots of them out there. Let's Encrypt does not provide a mechanism by which you can provide a CSR and get a cert. However, there are some third-party tools that do; see:

A problem with using Let’s Encrypt this way is that your certificate won’t be renewed automatically, so you’ll have to repeat the process frequently.