Measure up from the bottom of your post about 1' and mark a line. Now use your twine to make a level line across the 3 posts. This will be your base for measuring all other points for guide wire. Measure up 12" and make a mark in the middle of the post, and repeat as many times as necessary.



As you can see I placed a wire every 12". Use your 1/2" drill bit and drill a hole through the center of the post at every point you've made. depending on your layout (did you install the cross pieces?), and the height of your posts your number of lines could vary.



If you placed cross pieces measure 12" and 24" from the center point and make a mark. Drill those holes too. Place an eye-bolt, facing in, and put a nut on the outside face of the post. Thread the nut on just a few turns. Now thread the wire through one eye bolt and wrap it over itself, through the center post and to the opposing eye bolt. Cut off the excess wire and wrap the tail end around this eye bolt. Make sure when you do this you pull the wire hand tight. Don't go nuts yet and try to get it crazy tight.



Measure the distance between your posts precisely and cut some chunks of angle iron ~ 6" long. Subtract the thickness of the angle iron x2 from the distance between your posts; this will be the length of the pipe you need to cut. Now weld the angle to either end of the pipe and slip it in between the posts. Don't overly concern yourself with it falling out.



If you chose to install the cross pieces place a angle&pipe piece on either side of your cross pieces.



Now you can go crazy and tighten those nuts that tighten the eye bolts. The wire will be pulled and cause the posts to slightly toe in. This pressure will prevent the pipe&angle brace from going anywhere and will prevent your vines from drooping once they start to put their full weight on the wires.



*Note: I used stainless eye bolts and nuts. In a few years when the wire stretches out it will be much easier to tighten than trying to work with @$^%^%$ rusty nuts and threads.



*Note: It isn't necessary to install the angle&pipe braces, but I find worry there is the possibility of the tight wire and wight of the vines pulling the posts out of level, which means that I would never really be able to keep the trellis wires tight and the vines could sag and be damaged. Maybe I'm paranoid and over complicated matters but after 3 winters the wires still pluck like guitar strings.