The simplest modification of an existing model set is to remove some of the surface while leaving everything else unchanged. For example, you can remove the neck from the 'Animate' head by doing the following (Windows only since it requires the GUI):
> cd ~sdk/data/csam/Animate/Head
> fgrt meshops toTris HeadHires.tri noNeck.tri
> fgrt meshops surf add noNeck.tri tmp noNeck.fgmesh
> fgrt view mesh noNeck.fgmesh
Select the 'Edit'->'Facets' tabs then select the 'tmp' surface created above and the 'Color by surface' option, then mark the facets to be removed using the drag and fill actions described there. If you don't need to use the fill action, just the drag action, you can avoid converting the mesh to all tris above. When you're finished, click on the 'FGMESH' button and save back to the same filename. Now you can remove the selected facets:
> fgrt meshops surf del noNeck.fgmesh 1 noNeck.tri
Finally, make copies of all other 'HeadHires.*' files (egm,bmp,egt,fim,txt) and modify 'noNeck.txt' with an appropriate description and your new model part is ready to use.
If you're removed a lot of surface and are worried about efficiency, you can also remove vertices and UVs no longer associated with a surface:
> fgrt meshops ruv noNeck.tri noNeck.tri
In this case, you will need to re-generate the .egm file to reflect the new vertex list:
> cd ..
> fg3t ssm InternalBaseFace.tri -hires Head\\noNeck.tri