To make one, you'll need 1.25 yards each of two prints of flannel (or 2.5 yards of a single print, though I think they are cuter when the front and back are different). It's important to use flannel (and not just regular quilting cotton), because it's "grippy", and will help keep the baby from wriggling out. Any flannel is fine, but I actually prefer the cheaper quality flannel (like from Joanns) because it is stretchier. I even used a thrifted flannel sheet to make a couple for my own kid, and that worked great too.
IMPORTANT - prewash your fabric! I know some of you quilters out there are hardcore non-prewashers, but you really need to prewash flannel, because it can shrink and change grain a lot. If you don't prewash, you're very likely to have edges that curl after that first wash.
Iron your fabrics, fold each one lengthwise (right sides out), and trim straight across one edge (just one!) on each piece.
Now line up the cut ends and stack the two pieces on top of each other. You'll often find that one print is slightly wider than the other. Put the wider fabric on the bottom and the narrower one on top. Measure the width of the narrower fabric, minus the selvage. Mine here is about 20.5" across (folded, of course).
You want the blanket to be square, so double that measurement (20.5 x 2 = 41" in my case), measure out that length of fabric, and trim across the ends of both pieces of fabric at once. Since you started with 1.25 yards (45") of fabric, this means you'll be trimming off about 2" or 3".
Finally, trim off the selvages.
The next step is to pin the two pieces together, with the right sides together. At this point, you have 4 layers of fabric on your cutting surface. The two inner layers should be one layer of each fabric, with the right sides facing each other. Grab just those two inner layers at the corners, pick them up, and let the top and bottom layers unfold on either side. Shake it out, line up the other two corners, and pin around all four sides. I don't use too many pins, just one in each corner and 3 more on each side.
Now we're going to sew all around the edge, leaving an opening for turning. I like to leave the opening on a side that was along the selvage (the length-wise grain of the fabric), because it's slightly less stretchy than the cross-wise grain sides. But any side will work. Start a few inches below the middle, back-stitch a bit, sew all around, stopping about 6 or 8 inches from where you started, and back-stitch again. Use a wide seam allowance (anywhere from 1/2" to 3/4") - I usually use about 5/8".
Before you turn it right side out, trim the corners to reduce bulk. I'm anal, and I always trim a tiny bit more than straight across.
Now, turn it right side out, and use a chopstick (or whatever) to poke the corners out. To get nice edges, it helps to first iron the seams out flat. To do this, grab one layer of fabric about 4" from the seam. Shake the blanket until the layers shift, and set the blanket down on your ironing board. From the outside, poke around until the seam allowance is all laying down one direction inside the blanket.
Then iron the seam flat. You should be able to see in this picture that my seam allowance is pressed toward the lighter fabric. Do this to all four sides, taking extra care on the side that has the opening. Then pick the blanket up by the seam, and shake it out again to get the layers realigned. Iron all the seams again, and you should have perfect edges!
Now, slip-stitch the opening closed (sorry I didn't take a picture of that, but I imagine you know how to do it!). Or, if you're being lazy and plan to edge-stitch close to the seam, you can skip that step. For gifts, I try to use some sort of decorative stitching, and maybe use contrasting thread. Since the swaddling blanket itself is pretty simple, this is one way you can dress it up. Just be sure to try out the stitch on some scraps before you start sewing on the blanket - you don't want to have to adjust the width or length after you start! And be aware that some decorative stitches use a LOT of thread, so make sure you have a full bobbin and spool.
And there you go - an easy peasy swaddling blanket!
A variation on this is a single layer blanket - these are great for warmer weather (hello, summer - so nice of you to finally visit Seattle!). Just cut a square of flannel as before, and sew a narrow hem all around, as shown here. This one has mitered corners (I didn't make it - it was a gift for my daughter), but just plain corners work fine too.
an earlier post, (complete with a hilarious video of my husband demonstrating how to use them), so you can check them out there. If you want them to match the blanket you just made, start with an extra 1/4 yard of each fabric. After you cut out the blanket, use the leftover pieces to make the bindings.
Have fun making them - they are addictive! Oh, and if you make more than a few of these, be sure to clean out your sewing machine when you're done, because flannel sure does lint up your bobbin case in a hurry!