RTFI
Download the component, and the module. Install the component in the Joomla admin backend, install the module in the Joomla admin backend. Read the instructions fully and set up the paramaters in the components settings area as you would like them.
As time goes by - when you mark an order as shipped in virtuemart, JD Testimonials will send out emails asking for a testimonial. Check the Testimonials tab in the component menu regularly for newly posted testimonials and publish them if you want. Hope that helps.
Have to say that I've been using it on my live site for some time now and it is excellent and regularly (nearly daily) brings in new testimonials.