I often grow my tomatoes this way as an alternatve to canes as you can see here; (if you look close);
What you do is tie the string to the base of the plant, remembering to leave a big enough loop for the stem of the plant thickening. Also allow a bit of spare at the top to cater for the spirals around the plant.
Spiral the string around the plant as it grows,
To clarify that last bit when tying the string to the horizonal wire (at roof level) use a slip knot, i.e. one that is easy to untie to allow you to wind the string up the new growth. Once you have done tie the slip knot again (does that make sense ???)