In CWP.admin left menu go to:
-> WebServer Settings -> Select WebServers

after you save & rebuild the configuration varnish will be installed.
Varnish gets activated per domain and that means you will need to manually activate it for each domain.

How to activate Varnish per Domain?

In CWP.admin left menu go to:
-> WebServer Settings -> WebServers Domain Conf - (Select Username and click on the button Create Configuration)
Now from the dropdown select webservers you want to use for that domain, you will need to select some option having varnish.
recommend option is: nginx->varnish->apache->php-fpm for the best performaces.

Custom Varnish Configuration
Varnish configuration is in the folder /etc/varnish/
Main configuration is /etc/varnish/varnish.params

We recommed using malloc for memory usage on the servers with more RAM.
VARNISH_STORAGE="malloc,256M"

we recommended 1-4G depending on your available server resources.
VARNISH_STORAGE="malloc,1G"