Purpose, is to match the brass to your chamber. So next time all you have to do is neck size or partial FL size and your brass is fully supported in your chamber to enhance accurcy.
How to do it? Load some bullets and powder and fire it. I want to find a way to use fast powder like bullseye and a media to fireform it without shooting a bullet in my 300RUM.
Like Bigcountry said, simply load up a round, or shoot a factory shell through your gun, and you have a fireformed case. Also, when reloading it helps to keep a fireformed case, and a new factory round near. It can be used to check that your die is working properly, and that you have it adjusted correctly (with the use of a stoney point comparitor).
To fire form brass
