There are a few different ways to use a lemon squeezer, and the correct way will depend on the type of squeezer you have. If you have a handheld squeezer, you will need to cut the lemon in half with a knife before using it. Place the halved lemon cut-side down on the squeezer, and then press down with all your strength to extract as much juice as possible. You can then pour the juice into whatever container you need.
If you have a manual juicer, place the halved lemon cut-side down onto one of the cones and then screw the other cone on top. Again, use all your strength to twist and extract as much juice as possible before removing the lid and pouring it into a container.
If you have an electric juicer, simply place the halved lemon (cut-side down) onto one of the cones and let the machine do its work. The juicer will automatically stop when all of the juice has been extracted - no need for manual twisting or squeezing!