What Crazy Domains have omitted to mention is that they don’t really support Let’s Encrypt in ernest. Let’s Encrypt is supposed to be automatic: you shouldn’t have to muck about with CSRs or anything like that. You shouldn’t have to do anything at all - it should appear in your Crazy Domains hosting account by itself.
If you choose to do what Crazy Domains is suggesting, you would need to repeat the entire process every 90 days, since that is the duration for which a single Let’s Encrypt certificate is valid.
If you want to continue anyway, one of the easier ways to do what you are trying to do is to use a tool like ZeroSSL to obtain a Let’s Encrypt certificate from a CSR file. Their website contains some videos if you want to see how the process is done.