This instructable will show you how to make a "peanut shell" style baby sling for under $20, (even lass depending on your fabric. You can also find similar info on this site http://mykarmababy.com/pages/BabySlingPattern.php

Step 1: Supplies, Choosing and Treating the Fabric

1.5-2 yd. fabric (this depends on what size you're sling will be, which depends on what size you are)
Sewing Machine
Thread
Scissors
Pins
Iron

The first step is getting the right fabric, and there are a few things to keep in mind:
1. Don't get anything too stretchy, (if you do, the baby will sag down, and won't be safe and secure in the sling)
2. Great something that's washable!
3. Get a strong pattern that will hide stains well, (let's face it, kids are messy!)

Once you've picked out your fabric, be sure to wash it in mild detergent at least once.

Step 2: Determine the Size of the Sling

To make your sling the right size, measure a straight line from the top of your shoulder, across your body to your opposite hip, (in the same way a sash is worn). For example, my measurement was 27". Multiply it by 2, for me 54". Then add 3" to that measurement, for me, 57"
(HIP TO SHOULDER MEASUREMENT x2) + 3" = SLING LENGTH

Then cut your fabric so that it is 22" wide by the sling length. For example my sling is 22"x57"

Step 3: Flip and Iron

Flip your cut fabric so that it is "wrong side" up.
Iron thoroughly.

Step 4: The Folding Starts

fold the fabric along the "sling length" axis, (the long side)

Step 5: The Folding Continues

now fold up, along the 22" axis

Step 6: Cutting the Curve

Now measure in 3.5" from the top corner (where all the frayed edges are) and mark with a pin.
Then cut from the bottom corner (full length) to the top corner mark

Now open it up along the 22" axis to show the curve you just made, and pin up the curve.

Step 7: Making a French Seam

A french seam is a way to make a very strong seam without any unfinished edges.

1. Sew the curve you cut and pinned in the last step.
2. Cut away any excess fabric, leaving about 1/8" away from the seam, (be careful not to cut the seam, if you do, stop, re-sew that section, and start where you left off)
3. Flip the fabric "wrong side out" and iron the curve and seam
4. Sew the curve again, about 1/8" in from the seam.
5. Flip "right side out"
6. Great job, you just made a french seam!

Step 8: Sewing the Hem

While your sling is still "wrong side out" Fold the edges down about 1/4" and iron, then sew them (a zig-zag stitch is great here) as the hem.

Now you're done sewing!