https://help.bigcartel.com/basics/domains/dreamhost/#use-www

On the Dreamhost Side:

From the panel,  Go to your  Manage Domains Page and make sure the domain is DNS only. If you have hosting on the domain, use the “remove” button under the “web hosting” column:

Once the domain is dns only, click the dns link under your domain name and add these records:

Custom DNS Options:

Name: subdomain, leave blank for naked/root domain

Type:  type of dns record, such as A record, Cname, TXT

Value: Target

 Bigcartel only allows Cname sub-domain Records, you must get the record from their form, link listed above. 

On their dns page you will see this form, make sure to select Dreamhost for the DNS host:

Settings Needed for this setup:

Name:  www

Type:  CName

Value: the cname provided by the form

 Then you must set your domain to redirect to the www subdomain of the site. Go back to the Manage Domains Page and use the add button:

On the next page use the “Redirect” and redirect the domain to the www version of the site where the record is. This will cause all traffic to go to that record.

Note: This guide has been created to help simplify setting your domain up. The service you are pointing your domain to, DNS can/might has changed. Always check the hosts official page to verify the records.