How do you correctly pull the steel wire and crimp so it's tight enough for turnbuckle?
Without photographs or sketches, this is going to be more of a general answer than a precise solution. Focusing on the loose end, you have the cable passing through the sleeve, around the eyebolt and back through the sleeve, with the aforementioned seven inches extending.

I would use a simple ground wire clamp attached to the loose end, to add an extension of the same cable. This extension reaches to a location of some stability, a tree, a stake, the next post in line. The extension is reusable, not wasted.

Place a lever at the base of the stable post and attach the extension as low to the lever pivot as possible and as low as practical. Too low means insufficient travel, too high means insufficient force.

As the lever is pulled away from the loose end eyebolt, it will tension the extension cable and the original cable. Tie off the top of the lever to maintain tension (or use an assistant) while securing the sleeve with the tool.