help me on a trigger

I need to create a trigger that decreases on a "stocks" table the amount that have been inserted on the "tooken_from_stock" table.

the trigger should be something like:

CREATE TRIGGER triggerdecrease
BEFORE INSERT ON tooken_from_stock
EXECUTE PROCEDURE decrease();


then i need to create the procedure decrease that gets the inserted "tooken_from_stock.amount" and subtracts it on "stocks.amount "

can anybody help me with this please?

thanks in advance.