Start with the simplest bit and work up. Since you are not editing a
resource then use form_tag rather then form_for. Google and look at
the docs for how to use it (you will find many examples). Once you
have got a form to display then work out how to get the list of
products on it. Then work out how to get in the input fields, and so
on. Come back with questions when you get stuck.
Use a code control system such as git to manage your code so you can
easily experiment and go back to previous versions if you find
yourself up a blind alley.
Write tests as you go so that you know that the code works.
I presume you have worked through a number of tutorials on basic RoR
functionality, testing and so on. Even if they do not seem to be
directly applicable to your requirement you will learn a lot by doing