Please see our latest documentation here:
ECID library methods in a Safari ITP world
The diagram within the documentation explains the basic process and a summary of what is needed to implement this is included below:
Essentially, you'll need to work with Customer Care to set up a CNAME. This helps create a new first-party ECID cookie called s_ecid that is set server-side (rather than client-side). Once that's setup, be sure to upgrade to the latest version of ECID Service code.