There is not much to add actually. Create the new class, you can copy/paste the above example, just make sure you use the correct name for "user id" field (the one you used when building your table). Then open your model class (it will most likely be on "site" side, so something like <joomla root>/components/com_<yourcomponent>/models/<your_table_alias>.php), go to class declaration line and change the name of the class it extends to the name of the new class. Now, based on inheritance laws, whenever "_buildQueryWhere" is called, your new class will add "user id" clause to WHERE condition.
Hope this helps,
Michael