One thing I have noticed, is that the bigger candles smoke a lot more than smaller ones. I originally thought that the bigger the candle the faster the process, but no.
Every time I'd try to put the fabric over/near the flame (I put the edges of the fabric a few inches above the candle in the column of heat coming from the flame, if that makes sense?) it would waver like crazy making it impossible to singe it in the right places, and the smoke from the candle would also stain the fabric. So I went back to the smaller jar candles from WM, they are $1/each, so while they don't last as long they are inexpensive and don't get my fabric all smoky.
I think the thicker organza singes better without burning. I have both here, and it is much easier to work with. The thin kind seems to burn easily and doesn't shape up as well. Shame since the thin kind is rainbow and I really want to make a rainbow one! I am going to keep trying though.