How can I be summoned by Looking Glass Knight?

I heard you can be summoned by Looking Glass Knight to fight the player trying to kill it. Is there a specific way or location that I have to be in order to be able to be summoned by Looking Glass Knight?

From the wiki

To do this, place a red soapstone sign anywhere in the castle in one of the areas around the 2 last bonfires and in time you will be summoned. You are more likely to be summoned if you replace your sign every ten seconds or so, rather than leaving it alone on the ground. This also minimizes your chances of being summoned by a player. You don't have to place it in King's Passage.

It is also possible to get summoned by the Looking Glass Knight by using a cracked red eye orb.