To use Cloudflare, you need to add a Cloudflare account
Under your Cloudflare account, you need to add your website/domain
In the process of adding a website, you would be given the Cloudflare nameservers
Change your nameservers for your domain at your domain registar to Cloudflare ones (GoDaddy?)
Wait for few hours due to DNS propagation
Regarding the 520 error you managed to get, were the A records or ?
Also, may I ask if you have already checked with the steps as written from below article?:
Is the domain pointed to Cloudflare nameservers?
Have you checked your web server log files and firewall?
Do you use any other CDN with Cloudflare?
Do you use any CMS like WordPress?
Do you already have an SSL certificate installed at your origin/host for your Website(s)
(domains and sub-domains)?
Have you got CNAME or A records for your domains pointed to your origin/host IP address at Cloudflare dashboard?
Who is the hosting provider?
Are the needed one and compatible ports with Cloudflare openned at the Origin/Host/Server and apps running on them (and not on the incompatible ones)?
Is Cloudflare allowed to connect to your origin/host?