An Extra Link is a series of co-linked Link Monsters which connect one Extra Monster Zone with the other.
Control does not matter
The control of the Monsters, including the Monsters in the Extra Monster Zones, do not matter when creating the Extra Link, as long as there are a series of Co-linked Link Monsters connecting both Extra Monster Zones.
All Link Monsters that make up the Extra Link, and any additional Monsters co-linked with those Link Monsters, are considered to be Extra Linked.
V shaped Extra Link
You can create an Extra Link using fewer than five Monsters by Co-linking them diagonally, resulting in a "V" shaped Extra Link.
Controlling both Extra Monster Zones
Normally, you can only use one Extra Monster Zone at a time. But when both Extra Monster Zones have been bound together, you can Summon a new Link Monster to the other!
This results in an Extra Link, meaning your opponent cannot Summon Monsters from the Extra Deck.