The turnbuckles were only going to give us 2' of tensioning, so we had to do as much tensioning by hand first. This proved to be incredibly difficult! 200' of steel cable is very heavy! I took the free end of the cable and ran it thru the thimble that was placed in the turnbuckle. My brother went to the middle of the zip line and used a long branch to lift the line to relieve some of the weight pulling on it. This helped a lot. Then, I slowly inched the line thru the turnbuckle trying to get as much tension on the line as I could. My hands were killing me and my fingers were starting to cramp up, but we got a lot of tension pulled up. So, I quickly attached a cable clip to keep the line from slipping. Then I let my brother finish attaching the other clips while I rested my hand.
At this point there was still a lot of slack in the line, so we were a little worried. But as we started tightening the turnbuckles the line quickly rose. I learned from this that it is probably impossible to hand-tension a zip line that's longer than 200' unless you're Hercules, so, if I ever build a longer zip line I'll probably use some kind of cable ratcheting device instead of trying to do that by hand. Or, adding a few more turnbuckles might be another way to fix the problem.