Love the question and all of Lizzyastro's answers will work however I must ask Why the fishing line?
I think that fishing line helps the tulle gather a bit more, creating a greater 'flounce', and it just makes it look more volumized.
Hey, I don't know how old this qyuestions is, but I have recently went through getting all the info about doing a fish line hem. For starters you can easily use a sewing machine or a serger. Also, depends on what you are making this for, but organza, though a little more pricey, may give you a better result. My daughters do pageants, an the dresses can run up to $2500. so I said I have to learn to make them and use then and sell them. For starters if you like those kind of skirts they use for pageants called a cupcake skirt you can get the pattern from mypageantkid.com. But anyways, seems to me you will be using a sewing machine, so you will need to buy a rolled hem presser foot for your machine. they are about $10.00 on e bay where I got mine, and so worth it!! They are magical!! LOL!!! They will roll your baric giving it the small hem. BUt to do the fish line hem, you will push that through with your fabric through the presser foot. You do not have to thread the fish line through the machine holes, just along with the fabric. This will give it a small edge and the curl will be amazing. The weight you would use is based on how heavy the fabric is. When I do the chiffonpageant dress I use like 20-30 lb and when I do a casual wear with cotton I use 50 lb test line. Also, make sure you buy the cheap one at wal mart (or wherever you go, I know there they are $2.50) the type a fishman wouldn't go near. This is tough and obviuosly curly. Also CRAFTV, the fish line is used to give it that curly effect like yo uwould see on wedding veils, that is how they acheive that look. I have heard of it being called a lettuce edge, but from what I understand it does not have as much curl and they dont use the fish line. This is a great way to achieve many looks!! Good luck and please post how it comes out, would love to see it. I will post my daughter's new rock wear I am working on this week when I am done!
The easiest way is if you have an overlocker or serger - you just do a rolled hem over the tulle and fishing line together - you will need to use a cording foot which is designed foot so that you can feed the fishing line in as well as the tulle.If you do not have a serger then I would recommend you do a narrow zigzag over the fishing line as close as you can to the edge of the tulle with a standard sewing machine, essentially a "couching" stitch, to use the embroidery term. Sew slowly and you may need to adjust your tension so you don't bunch the tulle up. Again a cording foot would be a big help for this.Select a fairly heavy weight fishing line, I think the stuff I used successfully was about 1/16" thick.