Now that you've got your shirt, wash it in hot water and dry it. Then, break it down by cutting along all the joining seams... no need to use your seam ripper, just clip right beside the seam. IMPORTANT: Do NOT take out the hem or the sleeve hem. Just the joining seams. I like to cut out the placket as well, because it's porportionally too large to use on a little dress, and is otherwise useless... I do save the buttons and the collar, though this is completely optional. When you finish breaking your shirt down, you should have something that resembles this:
Set it aside, and start on your pattern pieces. You'll need a onesie or dress in the size you are wanting to make. This isn't an exact science... there's a little bit of guess work involved (especially if your babe is still in your belly!), but it's pretty simple. Take your onesie or dress and fold in half longways. Place the fold along the edge of your paper and trace around the collar (Make sure you trace the different openings for the front collar and back collar) to the seam of the top of the sleeve, then from the seam at the bottom of the sleeve to the point you want the bodice to end. I stop about an inch below the bottom of the sleeve, but you can adjust that according to the size you are making and your personal preference.
Remove the onesie and connect from the top of the sleeve to the bottom of the sleeve. I just eyeball it based on the way the sleeve is connected on the onesie. Use a grid to add a 1/4 inch seam allowance to joining seams. Make sure you don't add to the collar or to the sleeve. You're now finished with your bodice pattern!
Now, place your folded onesie on the edge of another piece of paper. Make a mark at place you stopped for the bodice piece. Decide how wide you want your skirt to open at the bottom and half it. I like for mine to be twelve inches, so I measure out six inches. Make a mark somewhere near the bottom of your paper, and use a ruler to diagonally connnect the top mark to the bottom mark. Now decide how long you want your skirt. This is completely up to you, but I like to go three inches below the bottom of the onesie... shorten or lengthen based on size and personal preference. Using a ruler, draw a straight line from the edge (where the fold was) out to the width mark. That's your hem. Now, use your ruler to add a 1/4 inch seam allowance to the side seam as well as the top seam. Do NOT add to the hem... and your skirt pattern is finished!
Now for the sleeve pattern. You'll need the same piece of paper you drew your bodice pattern on. Place the seam of the sleeve of your onesie along the sleeve seam you drew on your pattern. Trace the sleeve. Add a 1/4 inch seam allowance to the joining seam of the sleeve... and your sleeve pattern is finished!
Now cut out your pattern pieces and you should have three pieces that resemble this:
The hardest part is over! Now use your pattern pieces to cut out your fabric (you'll need two of each piece). I like to start with the skirt first. Fold the body of a shirt piece in half and place the fold edge of your pattern along the fold and the hem edge of your pattern along the hem. Pin in place and cut. Repeat.
I like to cut the bodice piece along the same fold right above the skirt, so that I don't have to unfold and refold. Make sure that when you are cutting the bodice piece, you cut a piece with a front collar and a back collar.
Now that you have all your pieces cut out, you are ready to assemble! (Here I switched to some pieces I had previously cut out... I ran out of green thread!) I like to start with the skirt. Take your two skirt pieces, pin right sides together, and serge or zig-zag the sides. Make sure to leave the top open! Turn right sides out and press the seams.
Now fold the bodice piece inside out over the sleeve piece and pin all the way around the sleeve. Serge or zig-zag. Repeat for the other sleeve, then turn and press.
Now switch to an elastic bobbin. (You can continue with a regular bobbin by doing a gathering stitch and then stitching in place, but it's so much simpler with the elastic bobbin and it makes for stretchier dress.) Pin your openings right sides together and stitch. Iron over the elastic to help it draw up. Turn your dress right side out and you're finished! You can leave it plain, or use scraps to embellish.
You can do a flower.
You can do a ruffle.
You can even do both.
You can even use the scraps to make matching headbands!
... and pretty soon you'll have a whole cluster of inexpensive, repurposed, adorable handmade dresses for a special little lady.