This is a simple process that can be done from your domain registrar.
Step 1. Login to your domain registrar and locate your domain name's "DNS Settings".
Step 2. Add a CNAME record with the host as "www" without the quotations and point it to the root domain you're targeting.
This process is usually propagated within a few minutes, however, for some registrars, this may take up to 24 hours to propagate.