Where can I define my widget's preference?

[gaoxiang]

I think I'm having trouble with documentions.  These documentations gives 
little information.  I have not find any tutorials or documentations on where 
and how my widget's preference should be defined so I have to ask here.
Please give some links or hints on how I should do these things. After my 
program finish, I will write something about the help I got to enrich our 
documentations.
Thanks.