Are you on the latest version? If so, then you just need to run
pihole blacklist domain1 [domain2 ...] (filling in the domains) to blacklist domains. If you are not on the latest version, you can update by running
curl -L https://install.pi-hole.net | bash.
Or, as supernova mentioned, you could make your own list/find one and add a link to it to the ad list file (make sure the linked list just contains domains separated by a newline).
Also remember that on client machines the domains may not become blocked until after a period of time, since they usually cache DNS records for a few minutes.