Combine milk and vinegar or lemon juice, stir briefly and the let sit until milk is well curdled.
For best results, make sure to bring your milk to room temperature first. If you don't, it will take the milk longer to curdle and sour.
A strong acid is preferred, if using lemon juice it is best to used bottled or a standard fresh lemon (Lisbon, Eureka, etc). Meyer lemons are not as acidic and may take longer to sour the milk.