- Login to your Namecheap account
- Go to Domain List (in the left sidebar), choose the domain that you want to use and click on Manage
- Click on the Advanced DNS tab
- Click on Add New Record in the Host Records section and select "A Record" from the dropdown list
- Fill out the form:
- Host: For a subdomain (e.g. "demo" or "www"), enter the subdomain only.
- Value: Enter the IP address of your server (e.g. "111.222.333.444", shown in Sites and Databases > Domains).
- TTL (Time To Live): Leave to the default value, or specify a duration in seconds.
- Click on Save All Changes to create the record
You might have to wait a little while before the new entry is active and fully propagated.
For more information regarding the procedure, see https://www.namecheap.com/support/knowledgebase/article.aspx/319/2237/how-can-i-set-up-an-a-address-record-for-my-domain or open a support ticket.