john, i've broken more double flaring tools than most people have ever seen
the harbor freight one is the only double flaring tool i've had success with, don't let the HF reputation scare you away. its way better than the ones from napa, oreillys, pep boys, and of course autozone, even though they all come in the same or better plastic case.
Double flaring is when you flare it once so it gets diamond shaped, then a second time to fold it in so its a regular flare shape.
the way you cut the tube is important too. the HF tube cutter works great, but has a short life. Get 2, after about 10 cuts its done.