The Press Pull tool in the Modify toolbar allows you to add another sketch to the face of your object and use the new parameters to either add more material or remove material from your model.
Once you have created and extruded your sketch into a 3D object, create a new sketch on the surface of the object you would like to create a new extrusion/extension or remove some of the material in the current object.

Once you have created the new sketch, select the face of the new sketch. It should turn blue, as shown below. Once it is selected, click the Press Pull icon in the Modify toolbar. When you have your selection and click the icon, a blue arrow should appear in the center of your selection.
Adding Material Using Press Pull
To extrude your new sketch and add material to the model, simply move the arrow in the direction that gives you positive numbers in the measurement box. When doing this, the selection should remain blue.
Removing Material Using Press Pull
If you want to create a shape within the model, such as a hole or an indent for a shelf, we will still use the Push Pull tool, but a different method than the above will be used. When removing material from a solid model, we will move the blue arrow in the reverse direction and negative numbers will appear in the measurement box. When doing this, the selection should turn red to indicate that material is being removed from the model.