Set a strainer or colander over a deep pan or bowl, supporting it so base of strainer is at least 2 inches above pan bottom. Line strainer with two layers of cheesecloth or a clean muslin or linen towel.
Scrape yogurt into cloth. Enclose strainer and pan airtight in plastic wrap.
Chill at least 24 hours or up to 4 days (see notes), pouring off liquid (the whey) as it accumulates.