Edit ```/etc/squid/squid.conf```, changing the values for:
```
acl localnet src 192.168.1.0/24 # Change to your own network
dns_nameservers 9.9.9.9 # DNS servers (change if you want)
acl localnet src 192.168.56.0/24 # Change to your own network
dns_nameservers 192.168.56.11 # DNS servers (change if you want)
```
and ```/etc/squid/whitelist.txt```, adding or removing domains as you wish. Any domains in this list will be reachable, any that are not will be denied.