I find that putting the shoes on, first, and stetching the elastics over my feet is helpful in making sure I have the proper tension and the right place on the shoes to attach the elastics. I then grip each elastic in place with the thumb and forefinger of one hand while I use the other hand to mark the positions of the elastics. I do this with the sharp edge of a piece of white soap (like a tailor does with chalk).
I also suggest that you equip yourself with a seam stripper. It comes in many sewing kits. If you botch your sewing job, you can use this handy device to rip out the threads so you can sew, sew again. Just don't repeat this too often, as you will also damage the fabric in that area of the shoe, and make it easier for your properly-sewn elastics to weaken or even pop off. (This happened to me once, in class. Suddenly, I felt my left shoe come loose while we were practicing frappes - I had struck the floor instead of brushing. Naturally, there was nothing I could about it at that point. The shoe stayed on - until I did a grande battement. I managed to punt the shoe clear across the room, just missing the instructor's head, and bouncing off the mirror.)[/i]