|My daughter in her Cotton + Steel sundress.|
Cut your yard to the correct dress length for the arm pit to knee measurement plus about two inches for the hem and elastic casing. In my case my fabric is 16 inches long by 24 inches wide.
Sew the edges of your fabric together to made a tube. (You will be sewing along the arm pit to knee length.) This will be the centre back seam of the dress. I used a 5/8" seam. I then zig zag stitched to prevent fraying and also trimmed with pinking shears. This is optional. If you have a serger or a serging stitch on your machine use that. Press your seam open.
Fold up your hem edge to the wrong side by about a quarter inch and press. Fold up once more and press. Top stitch along your hem edge. Ensure that your bobbin thread matches your dress fabric as you are working on the inside and the bobbin thread will be what you see on the outside of the dress.
Then repeat this procedure on the top edge of the dress. Turn fabric down a quarter inch on the wrong side and press and then turn down once more but this time make it a half inch or 3/4 inch depending on the width of the elastic you are using. Sew all the way around the top edge starting and stopping on either side of the centre back seam. Leave an opening big enough (about an inch or an inch and a half) to thread your elastic through the casing. If you are making fabric straps and you wish to catch the raw edges inside the casing you should attach your straps in this step (See more about straps below). Tuck your raw strap edges under the casing and pin so you can catch them in the seam. You will need to wiggle your safety pin past these edges in the next step.
Cut your elastic to the chest measurement you took of the child you are sewing for. Thread the elastic through the casing using a safety pin attached to one end of the elastic.
Overlap your elastic ends by half an inch and stitch them together with a square shape. Before you stitch your ends together make sure you did not twist your elastic in the casing as a twist will be uncomfortable for the wearer. Begin stitching near one edge, turn and stitch across the elastic to the other edge, stitch along that edge, turn again and stitch back to where you started thus making a square shape.
Tuck the elastic ends (now stitched together) into the casing and stitch your opening in the elastic casing closed.
At this point you could say you were done and have a strapless dress but straps will keep it from falling down (especially on wiggly babies). I used four 10 inch lengths of grosgrain ribbon for my straps but you could also make your own straps from the left over fabric you have after cutting your yard to the dress length.
If I were making my own straps I would cut them about two inches wide by the length I wanted and then press them in half. Open them up and fold each edge into the middle and press again. then fold it back in half along the first fold I had made and edge stitch along the edges.
To add straps, lay your dress out with the centre seam in the middle. Measure an equal distance on either side of the centre seam and pin your back straps to the dress. Match the location of the straps on the front of the dress. You may need to check the dress on the child to make sure the placement of the straps is right. To do this I would try the dress tube onto the child and mark where the straps should go. (Do not try the dress onto the child while you have straight pins holding the straps in place as it is likely the pins will poke the child.) I just took a guess at where I thought the straps should be and it worked out. My straps are about two inches in from the sides.
Once you like the placement of your straps tuck the raw edges under and sew to the inside of the dress sewing over the original casing seam so that it is not noticeable. Go back and forth to make sure your straps are secure (or catch them in the casing seam when you are making the elastic casing in the steps above).
Tie a little knot to finish the ends of your straps and then put the dress on the child and tie the two pairs of straps with a bow at the top of each shoulder.