Anyone who has tried straighten wire by hand knows how frustrating it can be.
One common method is to fix one end and tighten the other end in a drill chuck. Apply tension to the wire while turning the drill slowly. This works well but I find it can be tricky to detach the wire from the drill without putting kinks in it.
The method I'm going to describe is to build a jig. It works well and only take a few minutes to build.