Form "Warehouse" having fields

WH_Code

WH_Description

Form "Products" having fields

Product_ID

Product_Description

Now I have made a form "Stock_Status" having fields:

Lookup field WH_Code

Lookup field Product_ID

Stock_In_Hand

I have two more forms named "Arrival" and "Despatch" which contains information of Material arrived and despatched.

I understand that I have to create a function to update Stock_In_Hand field in "Stock_Status" form "on success" of form "Arrival" and "Despatch" but I am unable to understand how to proceed. Please guide.