I use something similar to what Bare Foot Farms showed in the picture to stretch welded wire. For a five foot fence, I took two 2x4's just a little over 5 feet long and used nuts,washers, and bolts to clamp it to the fence where I am pulling on it, then tie a rope to the top and bottom of the 2x4's. Then I use a rope with a hook on it to hook to that rope so I can slide the hook up or down when I tighten it so I can get the top and bottom equally tight. I find if I attach it like that chain in the photo, either the top or bottom is tight with the other loose so I have to go back and adjust the attachment point, sometimes a few times.
It is important to start off with the fence positioned right. How to describe it? If your ground is level, you need to get the first post vertical and make sure the vertical wire in your fence is centered on the post top to bottom. If the ground is not level but the slope is constant, then you need to start with the wire following the ground. If your ground has a lot of ups and downs in it, it is real hard to not have some sags in the fence.
I don't think I described that very well. Hopefully you can follow what I'm trying to say.