Put one cup of fresh salsa in a 32 oz, wide mouth canning jar.
Store loosely lidded jar in a warm place (80-95F) for 12 hours or more until it starts to smell "tangy" and begins to bubble.
Put a tight lid on the jar and store in the fridge.
Fermented foods should keep indefinitely but if it starts to smell funky, grow hair, get a slimy texture or goes a funny color, toss it!