When logged into your namecheap account, however over your username at the top -> 'Manage Domains' and click on your domain. When you've clicked on your domain, go to 'All Host Records' at the bottom of this screenshot which is on the left when viewing your domain:
Once you're on the 'Host Records' tab add a record like the following:
Scroll down to the subdomain area. The first box is the subdomain you want, in front of your domain e.g. if you want 'mc' then you'd only enter 'mc' in the first box. The 2nd is your server IP. The 3rd you should set to A address and the 4th you can leave blank as it will be automatically set when you hit 'save changes' at the bottom of the page.
Please note it may take up to 24 hours for your new domain to work to connect to your minecraft server.
If you do not have a dedicated IP, you will be required to enter your server port after your domain when connecting to your server in your minecraft client. E.g. mc.spartanmc.net:25578
If you have a domain from a provider other than namecheap then feel free to contact us via ticket or live chat if you need help setting it up to point to your minecraft server.