The first thing you need is to turn on your joist finder and make sure it works correctly. If the button does not turn on when the button is pressed, check that the batteries are not depleted.
Usually, the studs are located on the wall at a distance of 16 or 24 inches. So look for the stud of two feet. Attach the instrument to the wall and sweep along the wall with smooth movements.
When the stud sensor detects the beam, it will give a light or sound signal.
Now in that place where the device showed the location of the stud make a mark with a pencil. If your tool shows where the edge of the stud is located, then find both edges of the stud and make marks there. Drilling will need to be exactly centered, then our object will be as firmly attached to the wall as possible.
Further, to make sure that you have found a stud exactly, you should find an adjacent one that is 16 or 24 inches apart.
If you do everything correctly, your new shelf or TV will hang on the wall very reliably.