encyclo - 5:22 pm on Mar 12, 2011 (gmt 0)
The "how" really depends on the interface for the DNS settings at your DNS provider - which is usually either the primary hosting company or the domain registrar. Did you change the nameservers to the primary hosting company ones? Do they have a DNS section in their control panel? Anything to manage the zone file?
Also, do you know the target for the CNAME? Usually a CNAME looks like this:
gallery 10800 IN CNAME subdomain.otherprovider.net.
So you need to know what
subdomain.otherprovider.net should be.