Definitions
Domain registrar: This is the company you pay to register your domain. Godaddy is the most popular of these services.
Hosting provider / web host / host: This is the company that stores your website files and provides them to internet users on request. If you are using Run, Run is your web host.
DNS records: A set of records that tell the internet where to find your website, your email, and are also used for validation of various services.
The basics of setting up your Run site
After you've paid, there will be two DNS records shown on your Site Settings page in the Payments and Domains section. You'll need to enter both of these at your domain registrar. Run does not sell or host domains.
You also need to delete all A or AAAA records that existed previously (unless it's for a "subdomain" like help.yoursitedomain.com). The only A record should be the one we give you.
Once your DNS records are added at your registrar and your domain is entered correctly in the Payments and Domains section, our system will keep checking and once it detects that the domain is connected correctly it will say "Valid Configuration."
It may take several hours for the domain to connect in some cases but it usually happens within a few minutes after you add the DNS records at the registrar.
Troubleshooting
Check on your Site Settings page
Double check that your domain name is spelled correctly on the Site Settings page of your Run site.
Check DNS records at your domain registrar
Make sure you have both your A and CNAME records provided on the Run Site Settings page listed in your DNS records correctly.
Make sure you have only one A (or AAAA) record in your DNS settings (unless it's for a "subdomain" like help.yoursitedomain.com). Delete any extraneous A records. AAAA records also need to be removed. Running the HTTP-01 test here can generally tell you if this is the issue: https://letsdebug.net
Make sure you have only one CNAME record with the value of "www" in your DNS settings. Delete any extraneous CNAME www records, but do NOT delete any CNAME records with values other than "www".
Make sure your DNS records are applied to the correct domain name. Sometimes users have multiple, but similar, domain names at their domain registrar (i.e. jonforcongress.com vs jonforcongress.org). Make sure that the domain name you've applied your Run DNS records to, is the same domain as the one on your Run Site Settings page.
Check your Name Servers at your domain registrar
While DNS records tell the internet where to find your website and email, Name Servers tell the internet where to find your DNS records. Usually your Name Servers are pointed directly from your domain registrar to your domain registrar. But sometimes you may have set your Name Servers to point somewhere else - usually your previous web host. In those cases, you either won't be able to set your DNS records at your domain registrar, OR they simply won't have any affect. Make sure you know where your Name Servers are pointed, and that's where you'll need to edit your DNS records.
You can looks up where you domain's Name Servers are using this service - the area under Address is where you can find that information.
In cases where your Name Servers are pointed to a previous web hosting account, you might want to point it back to your domain registrar and manually move all of your DNS records back there. Be careful with this process:
Make sure you copy over any and all DNS records you might need
Your domain registrar probably won't let you update your DNS records until you've changed your Name Servers, so there's likely to be a small service interruption in email and website after you've transferred your Name Servers but before you've re-added all your DNS records at your domain registratr.
Advanced
Using multiple domains
If you want to use multiple domain names on your site, connect the DNS records of your main domain to Run, and then forward the other domains to your main domain, using the tools provided by your domain registrar.