Here is a list with all the detailed steps published by infinity free from the generation of the certificate to the CNAME in the control panel. (With the certificate, you can save it)
If what you want is to put one of infinityfree, I recommend that you follow the guide. Otherwise, you want to generate your own, you will have to go for an ssl provider.
If you have any, activated or half activated, I recommend that you delete it and start over.