30 foot streamers seem to work pretty well. You can go a little longer if you want, it can help reduce the odds of a mid-air. In the US, the RCCA standard is a 5foot string leader. Using a longer leader can also help reduce mid-airs, so you might want to try 10-15foot leaders.

Tie the string to the streamer. It doesn't take much of a knot to hold pretty well, I usually just go with a square knot. If you're worried about the knot tearing out of the streamer, you can put a bit of tape over the knot.

To tie the streamer to the plane, I just tie a small loop at the end of the string. Then I loop the string around the tail of the plane, and pass the streamer though the small loop, basically creating a loop of string around the back of the plane. It's easy and works well enough. Some guys put holes in their tails or use some other method to attach the streamer, it's really not critical.