This is another of these miscellaneous, only peripherally-baby-related craft ideas, but since I've gotten a request for it, here it is! (I suppose I can justify it as an environmentally-conscious idea: if enough people do it, we will save resources for our childrens' future!) Typical paper grocery bags are about 12" wide by 21" deep, with 7" sides; opened up, that's 38 " long by 21" high. This pattern is copyright to me (Jan Andrea) and is NOT for resale under ANY circumstances!
Whatever you get, you'll need a piece that's about 36" by 22" to make a standard grocery bag. So if you buy 44" wide fabric, you can make two bags with a yard, provided you make it a little shorter to accomodate cutting out straps; or if it's 60" wide, you can easily make two, or even three if you cut them shorter (again, for straps). Or you can buy webbing for the straps and have them the "correct" height. However, that size holds a *lot* of groceries, and may be too heavy when full, so you can certainly make them smaller if you want. I would suggest a depth of 18" ra ther than 24".
Of course, smaller bags are also quite useful. I have a ton of greyish twill that's 60" wide and just not selling as slings, so I made cloth bags for gift-giving this year. I cut the bag pieces as 30" by 20", with handles at 15" by 5" -- so a 20" long piece of fabric (20" by 60") yielded two finished bags. With a 5" gusset (see below), the finished bags are 10" wide by 5" long by 15" deep.
Straps can be made of self-fabric, very sturdy ribbon, matching fabric remnants, nylon webbing, or whatever else seems appropriately sturdy that you have handy.
Next time you go grocery shopping, get some of your items in a paper bag. After removing your groceries (hee!), carefully open up the seams. There should be one on the side, and another on the bottom, plus the gussets. For full-sized bags, use this as a template. For shorter bags, just cut off about 6" from the top of the bag (not the bottom, since the bottom piece will show you where the gussets go). Your pieces should be abour 36" wide by 18-24" long.
Sewing is really simple. First, sew the short sides. I would recommend a French seam (where the fabric is sewn first with a 1/4" seam allowance, wrong sides together, then turned so that the right sides are together and another 3/8" seam is sewn, enclosing the first seam) since it is sturdy and keeps the inner seams from ravelling.
Next, sew along the bottom. A French seam is good here, too. You can put your side seam wherever you want, but I would recommend positioning it so that it's about 3" from a folded edge. That will make it one of the corners of the bag.
To make a gusset, flatten the bag with the bottom seam on top, as shown in the diagram at right (which involves folding the bag into a weird triangular shape, but makes more sense when you actually do it). Then sew across the triangle so that your seam is about 6-7" long (mark it by sliding a ruler down until it reads 7"). When you turn the bag right-side-out, it will stand up like a paper grocery bag.
If you are using fabric for the handles, make a long tube about 1" wide. If you have a 4" by 18-24" long piece left from cutting out the bag proper, fold the strip in half (so that it's 2" wide by 18-24" long) and sew 1" from the folded edge. Don't trim the excess fabric; this will give the handle some strength and make it more comfortable to hold. Turn the strip right-side-out, and if you want to, topstitch the edges, or just along the center. Cut into two pieces, as long as you want the handles to be plus 3". If your fabric is too heavy to stitch as a tube and then turn it (that will be the case with many heavier fabrics, like canvas and home-dec prints), fold the long edges in by 1" (pressing if desired), and then fold that in half, and topstitch along the open edge. I usually do this "freehand" without pressing; heavy fabrics usually take a finger-press pretty well. You can also topstitch along the folded edge, and anywhere else you like; it looks fancier if there are more lines of stitching, but that's not strictly necessary.
Find the center of each long side of the bag. Press or mark or eyeball the location of the hem folds, about 3/4"-1" and 1.5"-2" (for the first and second fold lines) from the top edge. Pin the finished handles to either side of the center, along the second foldline, with the handles against the wrong/inside of the bag, so that there's about 4-5" between the ends of the handles.
Now, fold down the second hemline, including the ends of the handles, then fold the handles upwards; pin again. Stitch around the hemline, including over the handles. You will need to sew slowly over the handles, as the fabric there will be quite thick. You may want to reinforce the handle stitching again by making a square or X where the handles overlap the bag.
That's it! Now, go out and shop with a clean conscience. Just remember to bring your bag with you when you go shopping! (I always forget mine...)