Step 1, remove ammo from box. Step 2, place into gun. Step 3, shoot.
That's how you fire-form factory ammo.
Unless you are asking about special steps and prep-work. You can't do any of those with factory loaded ammo. Unless you pull the factory ammo apart, prep it, put it back together, but then that defeats the purpose of forming from factory ammo...
I was asking which was better, using factory ammo or reload 338 Lapua with the bullet in the lands. I read that land engagement is best for fireforming.
Your reamer is cut with a -.004 headspace. That will cause a slight "crush" on the shoulder of factory ammo if the barrel is chambered correctly. You should have no problems shooting factory ammo through it.