How to make a magic mixture:
• 120 ml vinegar
• 120 ml vegetable oil
• 120 ml shampoo or dishwashing liquid
• 2.5 liters of water

How to use
1. Mix everything together in a 5-liter bucket 
2. Place the bucket in the corner of the kitchen and wait a few hours until you notice a number of flies caught in the trap
3. Repeat until all the insects are gone