delete on cascade

Hi all,
I guess this is an already asked question, but I didn't found an answer, so
apologize me. Imagine I've got two tables:
skill(id,description) // primary key => id
family(id,description) // primary key => id
and I want to associate skills to families:
ass_sf(id_skill,id_family) // appropriate foreign keys

Tables are already created and the database is running. Now I'd like to
implement a delete cascade, thus when I delete a skill also its association
with the family must be deleted. I gues I should have declared the skill
table as follows:
CREATE TABLE skill
(
id varchar(20) on delete cascade,
description varchar(50),
primary key(id)
);

right? The problem is: how can I alter the table to add the column constraint
now, without redeclaring the table?
Thanks very much for helping me.
Luca