Done that more than once at work. I've also bent the lines with the nut on the wrong side of the bend. Now I tape them to the ends before I make the bends.
The neighbor just had a '64 T-bird that somebody installed front disc brakes on it and they only made single flares and they were leaking. Always make double flares on automotive brakes.
There are several different brands of flaring tools. Some of them work better than others. I have an old Snap-On set that works pretty well. At the phone company we had a neat "Imperial" flaring tool, probably from the 1960s that was my favorite. I have never found another one like it. I should have taken it as a retirement gift.
Just practice on some tubing. It's not hard but it takes practice to make a factory looking flare. You need to leave the correct amount extended from the tool to get the correct diameter flare. The instructions will tell you how much for what diameter tubing. Make sure the end of the tubing is cut perfectly square. Make sure the inside does not have any metal burs hanging in there. Then just barely chamfer the outside sharp edge of the tubing with a file. That helps form the rounded edge. Put a little oil on the flaring tool where it contacts the tubing, so the tool does not grab the tubing but slides on it. Make sure to wash all the oil off when you are done. It's a two-step process. You have to practice at how much pressure to put on the tool to make the correct diameter flare. Not enough pressure and the flare will be too small. You can check your progress as you go. Make sure as the tool contacts the tubing its centered. The second step finishes the double flare. If you really crank down on the tool the flare will be too thin and it will crack, then you will have a leak. When your done inspect your flares closely for cracks and always test for leaks after the first road test. This is not difficult at all, you just need to do a few flares on some scrap tubing, and you will be making safe and nice-looking flares.
