Go through the validation process with whoever you bought the the SSL Certificate through (The could be email or file upload verification). Once your SSL is verified you will go into IIS and "Complete Certificate Request" and upload the file you are given from the SSL provider. Then you go into the bindings and you bind the https to that SSL certificate.
Next, you go to your config files and change http -> https on all locations (Hopefully your link is configured in one spot, but occasionally people will hardcode links cause they don't know what the hell they are doing).
Finally, go into Cloudflare and turn on "HTTPS Always ON"