Combine chilis, garlic, sugar, and salt in a blender and blend on a low speed with some water until coarse and pulpy.
Pour the mixture into a glass jar and cover loosely with plastic wrap. Set on a windowsill for 3-4 days, stirring the sauce about once every day.
Once the mixture has settled into distinct layers with visible bubbles, remove the plastic wrap, stir, and pour into a blender. Add 1 cup of vinegar and and briefly blend on low speed for 30 seconds or until the sauce has a smooth consistency.
Strain the mixture into a saucepan, and cook on low heat while stirring until the sauce has thickened to the desired consistency.
Let the sauce cool and enjoy!