That depends on who or what you are trying to scry and how you are able to identify said target.

The most general-purpose scrying ritual involves a candle, a metal bowl, a pinch of salt, a splash of ink, and a drop of the caster's blood. One adds various target-specifiers at various steps of a short ritual, culminating in burning all burnable ingredients in the bowl and then pouring the ink over the ashes, filling the bowl with water, drawing and adding the drop of blood, and staring into the bowl until a clear image forms.

"These are all written for a human audience but I don't think the spell will care," Sherlock mentions.