I started ... and failed miserably
However, one concept I did learn, practice with a suitable diameter piece of plastic pipe. This gives opportunity to experiment with labium sizes and angles without risking what might have been an expensive horn.
As I'm sure you're aware, a gemshorn is basically an early ocarina, so the final pitch of the instrument is relative to the internal volume of the horn. Best start a bit too large and bring it up to pitch by filling it a bit with beeswax or similar.
Good luck
