How do you extend a lockout for a raid in World of Warcraft?

I have a friend in World of Warcraft who mentioned that they are farming old raids using a 'lockout', and they are sharing the lockout between their own characters and 'extending it'.

Look at the in game calendar I can see my lockouts, but I don't know how to "extend" them.

How do you extend a lockout for a raid in World of Warcraft: BfA?

After a quick google search of "world of warcraft raid lockouts", I found a nice video with detail here.

1. Using one character, go in and clear all the bosses except the final boss (or whichever boss you want to farm, leave it alive). We'll call this CH1
2. You create a custom group with that character.
3. Log out, and log into a second character, CH2.
4. Using CH2, apply to your custom group.
5. Log back into CH1, and accept your CH2.
6. Log back into CH2, and start flying to the raid.
7. Wait for CH1 to fully disconnect.
8. Walk into the raid with CH2, and click accept to take the same lockout as CH1.
9. From here, kick CH1 from the party, and clear the boss.

CH2 will now be fully locked out, yet CH1 will still be able to kill the final boss.

You can repeat this process with the same CH1, using a CH3, CH4, etc!

You can keep this going week by week by extending CH1's lockout:

Existing or recently expired IDs can be extended via the Social tab under Raid by clicking on Raid Info.
The ID of any instance to which a player is saved can be extended.
Doing so will extend the lockout period by the same amount of time as the original lockout
(i.e. extending an Ulduar raid ID will add 7 days,
a Heroic: Halls of Lightning dungeon ID will add 24 hours,
and a Zul'Gurub raid ID will add 3 days to the lockout time).
An ID can be extended more than once.