Step 1
The first thing you need to do is figure out how long your fabric needs to be. Take a tape measure and measure across your body from your right shoulder to your left hip. On your shoulder the tape measure should be right where a t-shirt seam would be. On your hip measure to the top of your hip (where your hand would be if you were standing with your hands on your hips). I hope that's clear. Now take that measurement, double it, and add 6 inches (this extra is for baby room, and seam allowance). So if you measured 22 inches from shoulder to hip your total length would be 50 inches. This is how long you fabric needs to be.
Step 2
Now is the fun part... picking your fabrics. Choose 2 coordinating fabrics and cut them so each is 21 inches wide and the length that you calculated in the first step. (This will make your final sling 20 inches wide. You can go slightly wider if you would like, but DO NOT go any wider than 24 inches when complete. It is unsafe for the baby at that point.)
Step 3
Now, position your fabrics right sides together and fold in half width-wise. Make sure all layers of the fabric are laying smooth. Now, fold in half again, this time length-wise.
Again make sure all the layers are smooth. You are going to be making another cut, so you really want all your fabric to be smooth and crease-free. (It has been brought to my attention that this photo makes it look like the material is narrower on one side. Sorry, that is just the angle of the picture. Your fabric should be a perfect rectangle at this point.)
Step 4
Using a pencil mark a curved line (like a half smile) along the side where all the loose ends of the fabric are. Notice that the curve straightens out close to the edge. Now, cut along that line through all the layers of fabric.
Step 5
Unfold the fabric, but leave the two pieces right sides together. Stitch each long side together. Trim edges to 1/4 inch if necessary.
Turn right-side-out and press seams.
Step 6
Put the open, curved sides together and stitch through all layers. Trim edge to 1/4 inch if necessary.
Turn so the other fabric is on the outside and stitch the curved side again. This makes a french seam. All rough edges should now be enclosed.
Take the flap that the french seam made, and lay it flat. Stitch it in place.
Step 7
This step is completely optional, I just like the way it looks. Stitch all the way around each side of the upper edge and lower edge of the sling. Fold in half length-wise and your sling is finished.
I hope this tutorial was clear. If you find any problems with it, or something doesn't make sense, please let me know so I can correct it or clarify myself. If you make a sling, let me know. I'd love to see the finished product.