There are a lot of different things that could cause your tailing loop when learning to double haul, it could be numerous things to do with your haul it could be to do with your basic cast.
I hate to say it but if you're unwilling to get some instruction you're off to a non starter, you will likely increase your learning time considerably and with all due respect you have a good chance of practising the wrong thing. *note in general - not directed at you!*
If you want one piece of advice just now, haul late, haul at the very end of the stroke. And practice by side casting so you can see the entire rod and line. Drop the line at the end of each stroke. Get Joan Wulff's book or DVD so you can see this exercise

And get a lesson!