Open the Effects palette and type Scribble in the Contains text box.When the icon for the Scribble effect appears, drag it to the auto-traced layer in the Timeline window. Or, if your layer is already selected, just double-click the effect to apply it.

After you apply the Scribble effect to the layer, the Effect Controls window opens, displaying the Scribble effect controls. Here you choose the angle, color, and style of Scribble you’d like to add to the mask.

Choose a mask option from the Scribble control that best suits your auto-traced image. Adjust controls by dragging the underlined value. To create fun, loopy lines, increase the Curviness value in the Stroke Options controls.

To animate the Scribble effect, set the Wiggle control to Curvy.

In the Time Controls palette, click the RAM preview button. (If the Time Controls palette isn’t available, open it by choosing Window > Time Controls.) Even though you didn’t set any keyframes for the effect, After Effects animates the Scribble effect because you selected a Wiggle control that randomly varies the effect over time.