Couple of suggestions that I found:
1. You will be using your tool to bell the line and flare the line. In both cases you will be tightening the tool against the jig. Don't overtighten the tool.
2. I found deburring the end helped make a cleaner looking double flare.
3. When your done and the line is installed flush it thoroughly to get any left over debris out. This could be from you making the double flare, from the brake line sitting around collecting dust, or from the manufacturing process.
Good luck
Paul