I made this tutorial a while back for an online forum I am on. This style of diaper elastic seems complicated at first but it is really quite easy when you try it! Just takes some patience.
Sooooo first sew all your layers together. I like to leave the opening for turning above my wing between the elastic and curve or turn of the wing.
Take your elastic and stretch your elastic from one point to another and add enough room to overhang when you tack it down. I actually use 2 hands for this but had to have a hand to take the picture. Do this for all of your elastic pieces.
Now fish your hand with your elastic inside the diaper and pin each end of the elastic from the outside where you marked earlier. Front elastic will take some finangling to get in but the legs and back are easy.
Now to topstitch. My machine allows me to move my needle right/center/left. I use it left to topstitch because I love my little red mark on my foot. It is the perfect distance to topstitch. I like to start by topstitching my opening closed but you could really start anywhere. Topstitch till you get to your elastic.
When you reach the elastic make sure to sew all the way to your mark. There should be around 1/4″ of elastic overhanging so that when you tack it down it wont come undone. At this point I set my machine to a 3 step zigzag. Lift my needle out of the fabric, lift my foot, and slide my sewing over. I found that using the side of my pressure foot against the edge of the diaper was the perfect distance to hit my elastic.
Now 3 step zigzag your elastic down. Once again I actually do this with 2 hands. One in front and one in back. When doing your leg curve you will probably want to stop and start a few times to make sure you are still catching your elastic.
When you reach the end of your elastic tack it down by going back and forth a couple times again. Then set your machine back up to topstich. You may have to lift your foot and needle again to get back in position.