Step 1: put 1-3 on the left side of the balance and 4-6 on the right side of the balance.

1. If the weights of 1+2+3 and 4+5+6 are not equal, there is no defective product. 1-6. At this time, the second step is: put no. 1 and 4 are on the left of the balance, and 2 and 5 are on the right.

① If the mass of 1+4 and 2+5 are equal, the defective product is No.3 or No.6. Step 3: put 1 on the left side of the balance and No.3 on the right side of the balance: if the mass of 1 is equal to 3, the defective product is No.6; If the mass of 1 is not equal to 3, No.3 is defective.

② If the mass of 1+4 and 2+5 are not equal, but the inclination direction is the same as that of step 1, it means that the defective product is 1 or No.5. Step 3: Put 1 on the left side of the balance and No.3 on the right side: If 1 and No.3.

③ If the masses of 1+4 and 2+5 are not equal, and the inclination direction is opposite to that of step 1, the defective product is No.2 or No.4. At this time, the third step is: put 1 on the left side of the balance and No.2 on the right side; If the mass of 1 and 2 are equal, No.4 is defective; If the mass of 1 and 2 are not equal, it means there is a defect.

Step 1: put 1-3 on the left side of the balance and 4-6 on the right side of the balance.

2. If the weights of 1+2+3 and 4+5+6 are equal, there is no defective product. 7- 12. At this time, the second step is to put 7 and No. The left balance of 10 and the 8th and right 1 1.

① If the mass of 7+ 10 is equal to the mass of 8+ 1 1, it means that the defective product is No.9 or No.2.. At this time, the third step is: put No.7 on the left panel of the balance, and No.9 on the right panel of the balance: if the mass of 7 and 9 is equal, it means that 12 is defective; If the quality of 7 and 9 is not equal, it means that 9 is defective.

② If the mass of 7+ 10 and 8+ 1 1 are not equal, but the inclination direction is the same as that of step 1, it means that the defective product is No.7 or 1 1. At this time, the third step is: put No.7 on the left side of the balance and No.9 on the right side: if the masses of 7 and 9 are equal, it means 65438.

③ If the quality is not equal and the inclination direction is opposite to step 1, it means that the defective product is No.8 or 10. At this time, the third step is to put No.7 on the left panel of the balance and No.8 on the right panel: if the masses of 7 and 8 are equal, it means that 10 is defective; if the masses of 7 and 8 are not equal, it means that No.8 is defective.

Generally speaking, the second step should be based on the results of the first step and the third step should be based on the results of the second step.

If there is anything you don't understand, I'll draw you a flow chart when I have time, which can help you understand.