I checked the instructions on DIYnet, I think that those are great for the wee baby girls, but you don't need to use a onsie or elastic cord to make dresses like the OP.
The first step is to figure out how long you want the dress and how long you want the top (t-shirt) of the dress. Styles change and you can trim the t-shirt shorter and use a longer skirt, or vice-versa. So trim the t-shirt if you want. I would remove the hem on the bottom just to reduce bulk.
Fabric amounts and prep: You need 45" wide fabric, as much as you would like the skirt to be long plus 2.5 inches or so for a hem. If I am buying fabric, I usually purchased 1/2 yard, by the time it is prewashed and straightened, it is about the right length. So prewash and straighten your fabric. Cotton wovens can be straightened by tearing the fabric from selvage to selvage.
Skirt seam and hem: Pin together and sew a seam down the selvage edge. Press the hem edge up 1/2", then 2", then sew the hem into place.
Attaching the skirt: Gather the top edge by stitching two lines of basting (long) stitches. The first should be 5/8" from the edge, the second 3/8" from the edge. Make your life a lot easier by sewing these lines of gathering stitches on the right side of the fabric, then when you gather the bobbin thread, it will be on the wrong side which will face you when you pin. Gather the skirt by pulling the bobbin threads until the skirt top edge is the same as the width of the t-shirt bottom. Fuss the gathers so they are even. If one thread breaks, keep going, but if they both break, start over. Pin the gathered edge to the t-shirt, right sides together. I usually divide the skirt into quarters, the top into quarters, then match them up with the seam at center back. I am a perfectionist.
Sew. Open up the dress and press the seam up if you want, or not if you are iron phobic.
Optional: If you want to add a applique patch as I mentioned in a previous post, sew this on the t-shirt before adding the skirt. I just cut out what I liked, used a glue stick to hold it on the shirt, then used a close zig-zag around the edge. The glue stick is any old washable school glue stick I can find around the house. This is a good way to hide stains!!!
Variation: A friend of mine likes A-line skirts on her t-shirt dresses. She cuts her skirts into a trapezoid shape. The top of the trapezoid is the width of the t-shirt, the bottom a few inches wider, the length about 9 inches. Add seam allowances, sew as above without the gathering steps.
HTH!
If you ever need a quick, fun, easy, **clean up your stash of excess fabric**, and attractive project...this is a great one. I read about this about a year ago and finally broke down and made one for my dd from a cheepy 3$ walmart tank top. It's SOOOOOOOO PRETTY and she is beyond thrilled.
it's a pretty pink tank with the skirt part serged on (but can be sewn on with a sewing machine) that is orange with pink "bubbles" and dragonflies. I had it made w/in 10 min. and it got rid of some of the over abundance of fabric I have 
