Going through The Curtain

Back before the Extinction, when The Curtain still separated the material and spiritual world, one needed either some dumb luck or very specific knowledge to travel between the two of them. In the first case, it was the lack of said knowledge that let one stumble through the tears in The Curtain by accident. The moment someone knows about The Curtain and the possibility of passing through it, they need to perform a ritual to be able to do so.

The mechanics of the ritual

Depending on the location, The Curtain varies in density. In some places, it is dense enough that no Spirit could enter the area. There are also places where it is thin enough that any stronger stimuli, a sudden burst of strong emotions, a catastrophe, or a big enough change in environment, can create tears in it. While for humans those tears are weak enough to only be able to peer through to the other side, some beings from the spiritual world can pass through them. If they are strong enough, they might even be able to bind themself to a place, an object, or even possess a person to be able to manifest in our world.
For the human to be able to pass, the tears in The Curtain need to be ripped open and stabilized.

Performing the ritual

Once a tear in The Curtain is located, there are a few rituals that come from different cultures and use different tools but all achieve the same results. The tear needs to be surrounded by geometric symbols, drawn, carved, or painted, even printed ones, Those symbols reflect the most often recurring patterns of The Curtain and will serve as a frame for the opening of the passage. Next, specific ingredients and tools need to be placed in the specific points of those patterns. While it is the candles that are the most used ingredients for this, their short "life" dramatically affects the length of time that the passage will be stable. Some cultures use specific sounds or smells, The Faceless organization found the most success with using certain frequencies (or simply the few Faceless Ones who specialized in opening tears).