First you need to cut off the sleeves directly after the seam.
Step 2: Trim and Pin
Next, turn the shirt inside-out. Then, if necessary, you need to cut off any excess fabric up near the shoulders to make it the right fit for a girl. After that pin the sides for a guide when you sew them. You can try it on and see how much you need to take in the sweater and then pin it in place.
Step 3: Sew sides
Next you need to sew the sides that you pinned, and cut off any excess fabric.
Step 4: Cut down middle and pin
Now in order to make it a cardigan, you have to cut directly down the center of the front. Then fold over the sides twice, pin, and sew the seam. Make sure you fold the seam so you wont see it, on the opposite side you will wear it.
Step 5: Fold and pin sleeves
Fold over the sleeves twice, pin and then sew them.
Step 6: Set and sew buttons
Turn the vest right-side in again. Find out how you want to space your buttons, and then sew them on.
Step 7: Make button holes
Next you will need to make the button holes on the opposite side of where you placed your buttons. Many sewing machines have button whole settings, but I find it just as easy to do it with scissors a needle and thread. You first take you scissors and poke them through the fabric.
Step 8: Size hole
Push the button through the hole you just made to make sure it fits.
Step 9: Sew button hole
Next you need to sew around the hole so it wont fray. To do this you need to do an oversewing stitch, which basically goes through the fabric and instead of going back though you go around the edge to hold the fabric, and then back though the place you started. if you don't understand you can look it up.
Step 10: Finished Button Hole
This is what your button hole should look like when you're finished.