At the end you can wrap the string several times around the post to get it tight first by the friction of the loops tightly around the pole and if you use two hands you can guide with the first hand and yank tight with the rear hand. But it is better to do with two people so one can hold the string and the other can tie the knot. For when to start, I think you were enthusiastic with only four leaves, and a little quick, though it probably doesnt matter that much as long as you were careful and you didn't wait until the plants leaned over too far which to me is when you are getting late. If the first string is around say 6 inches off the ground, there should be plenty of room for healthy 12-18" tomatoes to get weaved. If the plants are leggy you might feel for them, it happens to me ;-) Hopefully they will grow at a uniform speed which doesn't always happen if you mix varieties. But you can just come back later and slip the slower ones in.

PC

see this video from Univ of Maine: https://www.youtube.com/watch?v=XSf3aSj46jo

This is how I have been doing it for years. While it isn't exactly a weave, it supports the plants and does a great job. https://www.youtube.com/watch?v=8ZHEL0_uBww