Are you starting it like a french braid or just at the nape of your neck? If you don't start it like a french braid it can help to put a rubber band in your hair then it's easier to pull the top part of the fishtail tight and snip off the rubber band when you're done.
Yeah, that's what I've been doing, but the top part still looks bad. I think I just have to keep practicing. Once I get the hang of it, I'll teach myself how to french braid my own hair, too.