On the Geometry toolbar, click on the 
button, then the 
button.
Select a curve which will be the outer loop of the new surface.
Select a curve that will be the first inner loop of the new surface or press enter.
If you pressed enter, go to 7.
Select additional curves for inner loops.
Finish by pressing enter.
Select a surface or press enter to fish.
If a surface was selected a trimmed surface is created otherwise a planar trimmed surface is created.
The resultant surface will be highlighted.
