how do you pull out a tree for a root flare and how do you put it back in?

What are you asking? Where would you like to pull the tree out? If you mean: When buying plants somewhere in a nursery, you can carefully scratch the dirt aside to see the roots.

on this site (train/rootflare) instructions show that rootflare is done by digging the tree out cuting the vertical roots and puting the tree back in, but i am kinda Skeptical about doing that because it is like repoting but whit an extra damage to the tree because of the cut of roots

Ah, now I see what you meant with "pull out a tree for a root flare"

This is a normal practice when repotting trees, but only then. And not too much; depending on species you only prune away 20-50% of the fine roots.

on the species guide on this page dosent say how much roots you should cut (20-50%) for an oak tree, do you maybe now?

we will need to know how big/old the tree is,and pics would help even better