how to create plugin

[adhirath]

hello friends ,
    i want to know how to create plugin for wich i have changes into vm2
                   i am new in virtuemart i have insert 1 downloadable  file into product field when customer purchase product this file link is send their email with product details also user can download directly from link .

but now i want to plugin installable this changes for third party  so please Help Me i haven't idea how to plugin not overwrite third party's virtuemart just this change plugin
plz help..!   
mail :-info@adhirath.in
www.adhirath.in