Extract works with a mask that you paint or apply to the model. It creates a new subtool (when you press Accept) but it uses the same topology so if your mask has a lot of detail (such as with lace) then you need a dense mesh to start with. Otherwise the mask won’t hold the details and neither will the extracted mesh.
http://docs.pixologic.com/user-guide/3d-modeling/modeling-basics/creating-meshes/mesh-extract/
There are various settings associated with the Extract function which you can read about at the bottom of this page:
http://docs.pixologic.com/reference-guide/tool/polymesh/subtool/
To get the hang of how Extract works I suggest you just paint a mask on a sphere to start with and experiment with the settings.
To mask a model in a precise way so you can create the lace effect you are after you will need UVs on your model. UVs are needed when you want to put a 2D image on a 3D object - the UVs give the information about how the 2D image is attached to the 3D object.
Although ZBrush has various options for UV mapping they may not give a good result for what you are trying to do, so you may need to look at creating the UVs in another program. What you need is a UV map that you can easily create a lace pattern for, without too many seams or distortion.
You’ll need to create a black and white version of your lace pattern to match the UV map. You can do this in an image editor such as Photoshop. Once you’ve done this then you can import it into ZBrush as an alpha (press the Import buitton at the top of the Alpha palette). It can then be used to mask your model. With the alpha selected in the Alpha palette you press the Tool>Masking>Mask By Alpha>Mask By Alpha button.
Once your model is masked you can try the Extract but I do suggest you start with simple stuff until you’re familiar with the whole process.
