Grape vines can grow in erratic directions as they reach towards the light. Sometimes, they might grow for a 12 inch stretch in a perfectly straight line, but then they will change and grow another way, leaving you with a crooked vine. You can straighten your vine by adding a little tension and by pruning your vine to grow straight.
Set your grape vines about 6 feet apart from each other in a row under a wire support trellis. The top wire of the trellis should be about 6 feet high from the ground. This will be the highest your vines will grow.
Allow the grapes to grow unhindered the first year. The idea, according to Peter Christensen of the University of California, (ref 2) is to let them grow the maximum number of leaves which will then support the maximum amount of roots per plant. They shouldn't grow too much, but keep them off the ground by tying them loosely to the trellis if they do have a lot of growth during their first year.
Tie a section of twine about 2 inches from the main growing stem of your grape vine. Make a simple knot, but do not make it so tight that it restricts the plant. Take the other end of the section of twine, and tie it off straight above the plant to the top wire of the trellis, again with a simple knot. Make sure that there is only a slight tension on the plant, or you will end up pulling the plant up by its roots.
Prune off any side shoots that extend outwards from the sides of the main growing stem. You want all the energy of the plant to go into forming a straight stem. Once the stem reaches the top of the trellis, you can allow it to branch out on either side, but that will occur during the second or third year of growth.
Adjust the tension on the string as the vine grows upwards. Every few weeks, you will need to re-tie the top of the twine as the vines gets longer the section of twine needed will get shorter. Each time, make sure there is only a slight tension pulling the vine upwards.