This tutorial will teach you how to alter a shirt to a smaller size. The shirt used in this tutorial is a polo, but any shirt will work.
Step 1: Pinning
First have the person put the shirt on inside out and then pin along the seam to the desired width, we took off about an inch on both sides with the shirt used in this tutorial. Don't forget to do the sleeves, making the sides smaller will affect the laying of the sleeves if you do not adjust accordingly. After you have pinned all the seams that you will be altering have the model carefully take the shirt off so that no pins come out.
Step 2: Sewing the Seam
After you have finished pinning, set up your sewing machine. Do not cut the old hem yet, you want to make sure the shirt fits correctly after the alter. Leaving the old hem will allow you to adjust if there are any mistakes. Follow along your pin line, not forgetting to back stitch at the beginning and end, all the way down the shirt's side. Repeat on the other side as well.
Step 3: Trying It On
Have the person try their shirt on to make sure the new seam fits well. It is a good idea to have them move around and be active a bit to make sure the new seam is not too tight and hinder movement. The shirt we used for the tutorial had slits at the sides. If your shirt has this, you will need to cut the seam up under the slit to give you room to pin. After you do that, pin the slits to the proper position. You will then just follow your seam line to hem them.
Step 4: Trimming
Now that you have finished altering to the desired size you can trim the excess fabric. First, trim the thread tails close to the fabric. Now cut along the new seam about 1/4-1/2" from your seam. Trim off any extra fabric along the slit. Iron the hem flat.
Step 5: The Finished Product
Have the person put the shirt on one more time and make sure it fits well and there are no weird laying places or thread hanging around. Hope this tutorial was helpful for you! Peace out buttercups.