When you setup your domain with Cloudflare, it will import your current DNS records then all you need to do is change your nameservers to the Cloudflare ones, and that's it, it's a very simple procedure.
A tip for anyone in the future. If Xenforo is the only thing on the site, you don't need to edit .htaccess. In the admin panel:
Setup -> Options -> Basic Board Options
Change the board URL in the admin
Make sure "Enable board URL canonicalization" is checked. This will automatically redirect versions of your domain that don't match the board URL setting.