Call Procedure from a Button

Question

I am trying to call a procedure from a button that will work out the total of an order and display it in the total field in my master block.
I am unsure as to how to call the prodcedure from my button. I have written the procedure in the PL/SQL Stored Program Units part like it says in the help file but am now stuck.
I assume I use a WHEN-BUTTON-PRESSED trigger but what do I write in the trigger?

Answer 1

What about placing outputs from the procedure to fields on the form?
Here is the procedure I have created:
procedure vatTotal (Order_no IN number,Total2 OUT number)
AS
CURSOR c_vatTotal is
select order_line.quantity, gre_product.cost from ord, gre_product, order_line where ord.order_no = order_line.order_no AND
gre_product.prod_no = order_line.Product_no;
Total number;
begin
Total:= 0;
for c_record in c_vatTotal loop
Total := Total + c_record.quantity * c_record.cost;
end loop;
Total2 := Total;
end;
I believe this is correct. It compiles.
I tried using Total as an out parameter but received the "duplicate fields in RECORD, TABLE or argument list are not permitted" error.