I started with two shirts (which I forgot to take pictures of before I cut, so here they are with the least cutting done).
The blue shirt will be the body of the jacket. Carefully cut along the seam up either side and around the sleeve so it looks like the above picture. It should only be connected across the shoulder seams. Flip the back part of the shirt up and out of the way.
Next, put the shirt over the head of the person you are making this for. Using chalk, mark a new seam line along one front side. This makes the shirt smaller in the shoulders, arm holes, and body so it will fit right. Cut, then fold the shirt over and use it to cut the same thing on the right front side of the shirt.
Fold the back part down and cut the sides using the front as your guide. Turn the shirt inside out (right sides together) and sew from the bottom edge up to where the arm hole is on both sides.
Following the original seam lines, cut the sleeves off of the second shirt (see first picture). I used a black shirt with white polka dots. It was originally a 3/4 sleeve shirt for a 12 year old, but the length is perfect for my 2.5 year old.
Lay the sleeves out flat next to the body of the shirt. Now you need to make them fit into the arm holes on the blue shirt. So draw a chalk line from where the arm hole is and taper it down to the ends of the sleeves like in the picture above (this will also narrow the sleeves a bit). Sew on the chalk line and trim excess fabric.
Now sew each sleeve to the bodice. Turn the shirt inside out. Turn the sleeves right side out. Insert the sleeve into the arm hole and match the bottom seam on the sleeve with the side seam in the shirt. It will look something like this:
Sew all the way around the circle. Trim any excess fabric. (I had to fudge mine a little because the top edge of my sleeves were shaped funny.) Repeat for the sleeve on the other side.
Okay, now we're going to make the pocket. Get a piece of scratch paper and cut it into a shape like this
Lay it on the shirt to make sure it's the right proportion. No secrets here, just eyeball it. But DO make sure the pattern is straight.
Lay your newly-made pattern onto the second shirt and cut TWO pieces. While I was working on this part, my daughter asked me if I was making underwear. HAHA.
I can see what she means. It DOES look like undies!
Anyway, on with the instructions. Now flip the two pieces so they are right sides together and sew around the edges, leaving a little space to flip it right side out. You may need to clip the corners of this to get it to lay flat when turned right side out. Iron it also to get really crisp, flat corners.
Pin the pocket to the front of the shirt and sew along the straight edges. Don't sew along the two curved parts---that's where your little one will put her hands in!
Now it's time to make the hood. Start by measuring the neck hole of the blue shirt. Then cut two pieces shaped like this:
The top straight edge should be 1/2 the distance of the neck measurement you just took. Flip the two pieces so they are right sides together and sew from corner to corner along the curved side.
Next, you're going to make a binding (shown above) for the edge of the hood. Cut about a 2 inch strip of fabric long enough* to go all the way around the opening of the hood. Fold it in half and press. Bring each edge up to the middle crease line and press. Make it look like the picture above.
Put the edge of the hood inside the binding so that it's sandwiched in there and sew the binding on.
* If you can get a long enough piece, make it so it will go all the way around the opening to the hood. I didn't have enough fabric, so I made two separate pieces, then sewed them to each side of the hood separately BEFORE sewing the two hood pieces together. Here's a pic if I'm not being very clear:
It will look much better if you can manage to get a long enough piece to just go all the way around the hood.
Well, now that that's clear as mud, let's move on. Attach the hood to the neck hole of the shirt, first lining up the hood so it's centered with the shirt. I lined up the back seam and the middle of the tag in back. Then sew it down.
Now all you have left to do is clip threads, adjust the bottom hem if the shirt is too long (I didn't need to) and then step back and admire your handiwork! Here's the finished product:
I guess she likes it: