Things to do first:
change the admin password so other people can't logon and disable parental controls;
change wifi password on router (and on all your wifi devices) so others can't bring in an unknown device and access restricted content; or put in an access list to only allow defined mac addresses;
keep both passwords in a safe place;
if you want 24hour restriction, it won't matter about the router's clock, otherwise set it;
Just reading through the manual:
enable parental controls
turn on content restriction
select blacklist (this blocks devices from accessing restricted content)
the router can scan for wifi devices. For each one to be controlled, you want to set 24hour control and you want to restrict access to www.youtube.com.
It gets a bit hard to describe just from the manual. Do you have one?
At the end, you want to save the config onto your computer in case you have to factory reset it or it breaks.
