You want to use flexible ssl/tls for starters, doubtful it will work otherwise. Log in to cloudflare, choose domain, then SSL/TLS and see if encryption is set to flexible. See what that gets you, though it can take 15 mins for effects to show up. As long as the server can be reached cloudflare will try and match a certificate so lemmy gets served, as long as the server is set up correctly and the ports etc. are correctly forwarded and open