Because there aren't many patterns for a pillow case dress, you can just make your own pattern. Don't panic, I'll show you how. First, get a dress that fits (sleeveless is best), or is maybe just a touch too big. This will be your pattern. Fold your fabric in half and lay your dress on the fabric. If you are making a diaper cover too, be sure you lay those pieces out as well. I found my diaper cover pattern over at "Make It and Love It".
Start with the top of the dress (the shoulders) at the top of your fabric. This will allow you enough extra for the seam allowance at the top. Then just cut a rectangle, the same width as the bottom of the skirt.
Next, you are going to cut out the arm holes. Again, don't panic, you have a pattern. Just slide the dress over so the top of the shoulder of the dress is touching the top of your fabric, and the under arm is touching the side of the fabric. Now, carefully, without cutting your dress, cut out the arm holes.
Do that same thing to the other side. Your pattern is cut out! Now, the real fun begins!
Top stitch or serge the top of the dress pieces and the arm holes.
Now, lay your two pieces side by side, sided touching and pull out your jumbo rick rack (or ribbon, or whatever you are going to embellish with). Lay this across both pieces - one long piece - and pin in place.
That is my little secret to making these match up on the dress, and trust me, I've have my share of ones not match up! If yours don't match up in the end, don't worry, this dress is going on a moving baby, and she's not gonna care :)
Top stitch this onto your fabric, then with right sides together, sew the dress sides together, making sure the rick rack matches.
Then, using your bias tape, pin it around your arm holes. It's kind of like filling a taco shell, put the dress fabric in between the two layers of bias tape and pin. Do this to both arms, then top stitch in place. Bias tape is kind of stretchy, and will form around the curve of your arm just fine.
Fold down the top of the dress to form the casing for your ribbon, which will become your dress straps. How far you fold it depends on the ribbon you will be using. I folded mine down about 1 1/4". Then stitch 1/4" from the bottom (serged) edge. Do this to both sides.
Now, it's time to hem your dress. I serged the bottom of the dress. You can zigzag the bottom too. Then I fold the material up about 1/4 to 1/2" and topstitch in place. I used a zigzag top stitch, but you can use a straight stitch, or a decorative stitch too. Your sewing is complete!
Only one final step, and that's to run the ribbon through your casing at the top of the dress. Start at the left hand side back of the dress, coming out the back right side, then threw through the right side front of the dress, coming out the front left side. Tie the two ends of your ribbon is a bow, leaving enough room for little arms to fit into and give a little extra for some growing room. This dress can be a sun top with shorts next year.
Admire your work and go eat a cookie! You deserve it.