I found this plaid men's button-down at Goodwill for $1 and figured I could upcycle it to fit me. I don't know why I felt the need to buy it, probably because it reminded me on Liam Payne :), but here it is:
Before & After
I cut out the shoulders first but you can do it afterwards too.
Fold the shirt in half with the front on the outside (see picture). Make sure you line up the seams where the arms meet the shirt. Then draw a line staring at the collar and going all the way to the seam where the arm is.
Cut out on the lines you just drew.
Now it's time to fit the shirt:
Turn the shirt inside out, then put it on. pin the down the sides and arms to fit you.
Now just sew where the pins are.
Then flatten the seam and iron it down.
Sadly, my shirt was still too long. Next comes the hemming:
I took about 1 1/2 inches the bottom.
Pin it down.
Now you have a finished, fitted shirt!