To scratch on a pioneer CDJ turntable, first select the sound you are going to use. Observe the position of the black line on the turntable in relation to the sound you've selected. Understand how much room you have to manipulate the sound and position your fingers accordingly. The platter will move in accordance to the black line and your finger. You can also lift your hand off the platter in the play and pause mode to let the sound move itself by its own accord and at its own speed. Move your hand in relation to how much of the sample you'd like to use. The initial position of your hand will depend on where you're most comfortable placing it. Move your hand with the platter for full control over the speed of the sample.