With the face selected, select the press/pull option under the gear icon. Type in a value in millimeters or drag the arrow handle to tweak the size of the shape.
Now select the object you want to become the part first and then select the object you wish to be used to cut (subtracted) your part. You can hold down the shift key to select multiple objects. Remember, the first object that was select will be the main part, everything select after that will be subtracted from the first shape.
To accept and apply the subtract boolean operation, click on an empty area on the grid.