This makes a patch pocket with a built-in flap, which allows you easy access to what's inside without having to fuss with a zipper. It's as secure as a zippered pocket, but without the hassle of sewing in a zipper.
I make my pockets about 9" square. This is big enough to hold a couple of diapers, along with my wallet, keys (when they aren't clipped to my sling's rings), wipes, and a toy or two.
I start with a piece of the fabric from the sling that's the width of the fabric (usually 26-30") and 10" tall. It actually works pretty well just to cut your sling fabric about 10" too long and include the pocket piece (that extra 10") in your initial hemming, then cut it off. Or you can hem the pieces separately -- it's up to you. I'm showing this hemmed separately in the photo below.
Hem both short edges of the pocket piece.
Fold one short edge down so that the hem is facing up (wrong side up, against the right side of the rest of the fabric) by about 3-4". If your pocket piece is about 28" long after hemming, 4" works very well.
Next, fold the bottom half of the piece up, so that the whole thing is about 10" tall, and fold the other edge down 4" again. This means you'll have four layers of fabric at the top. The hem will not show in this flap.
The next photo just shows the pocket ready to be sewn on the sides.
Sew both sides of the pocket, about 1/2" from the edges. Be sure to backtack at the start and finish of the seamline. You will not need to do any sewing on the top or bottom because the folds make the edges there.
After both sides are sewn, you'll turn it right-side out, starting with the bottom part. In the photo below, the top flap is now folded outwards on the underside of the pocket. Use a point turner to make nice points at the bottom of the pocket.
Now fold the top flap towards the front of the pocket, and use a point turner to make nice sharp points on the upper corners, too. The pocket is now ready to be sewn onto your sling.
Pocket placement: I sew the pockets onto the "wrong" side of the sling -- the side that will be facing out in the tail. You might want to thread and try on the sling before sewing on the pocket, so that you can place it where it makes sense to you, but I generally center it horizontally (so the pocket is in between the two edges) and have the bottom of the pocket about 4-6" above the end of the tail. If the sling is made to be mid-thigh length, that usually puts the pocket at about hip level. If your sling's tail is longer, you'll want to place the pocket higher, since if it's too low, it'll bang against your legs as you walk.
If your fabric has a "right side" and a "wrong side", I think it looks nice to make the pocket so the right side is showing, and sew it to the wrong side. Both sides will show when the sling is worn, of course, so it's best to use a fabric that doesn't have right and wrong sides, just so that you avoid this problem.
You can sew the pocket down on all four sides, or if you'd like to make it a double pocket (so you've got the outer pocket with the flap to keep your things secure, and an open-top inner pocket for things that you want quick access to), you can sew just the sides and the bottom, leaving the top open. Just be sure to reinforce the upper corners so that the pocket doesn't start to tear off with use.
When you're done wearing your sling and want to put it away, you can fold it up inside the pocket and use the inner flap to close it. Lay the sling down on the floor, and fold the tail into thirds, so the pocket is on top. Pull the tail up and out of the way, then fold the sling body into thirds. Put the folded tail on top of the sling body, then fold the whole thing so that the sling lies underneath the pocket. Open the zipper, and reach inside the pocket. Grasp the whole sling from the inside of the pocket and pull. This will pull the sling into the pocket and turn the pocket inside-out; then you can fold the flap around the open top and voila, sling packet!
I know that's not very clear, so if you're not on dial-up, click here for an animated gif showing how to do this (335 K).