I was planning to use it, but would like some guidance so I don't mess up, such as what to rub with, when the best time to rub the paint is, how to rub properly etc.

Tom ,, I haven't used the Testors brand but by the sounds of it,its just like the Humbrol Metalcote colours which are exactly the same as the Testors paint from the way you have described it .
I have used Humbrol's Polished Steel many times so i may as well tell you how i use it .
First of course i put some on a plastic painting tray ,then thin it out with some thinner to smooth it out ..makes it much easier to cover with a smooth finish .
Then i wait till its totally dry and then polish it .
I Q tips and napkins and the polishing wheels on my Dremel at the slowest speed .
Thats pretty much it . the more you polish the brighter it gets but you dont want it too bright cause then it looks more like custom work on a Harley

Phill