Hello
Are you sure that the sketch of your extrusion does not extend beyond the edges of the surface? Otherwise you have to select the surrounding area as well.
You create a plane at a distance X from your surface
On this plan you draw a sketch
You project your sketch onto the surface via the surface sketch selection
Then you extrude or you remove material
Take a screenshot of what you want to do and I'll give you an example
If you take a good look at my creation tree you should understand the procedure to follow
@+;-))
see the attached file eb SW 2012
I made a sketch offset to visualize the extrusion of the shape of the lettering
@+ ;-))
piece1_lettrage_pour_arnoob.sldprt
Here are the personalized tutorials^^
View this image Material removal
and the attached file in SW 2012
here I think you have enough ref now to do what you want
@+ ;-))
piece1_lettrage_pour_arnoob1.sldprt
@ Arnoob
Write down the answer that suits you in solved
@+