I'm an amateur seamstress. I'm self-taught. I just kind of do it. Technicalities are not important to me whatsoever. So, bear with me.
I took an old shirt that was a little too small for me.
Using straight pins, I marked the center of the shirt and cut it.
Then, I sewed about a 1/2" seam on each side of the front.
I cut off the sleeves a couple inches longer than I wanted them.
Using some of the extra fabric, I made a little strip of fabric. I sewed a little tube for each side. The seam went on the back.
I found some cool buttons in a button box left for me from my grandma. I LOVE them. I rolled up the sleeves twice, wrapped the strip of fabric from the inside to the outside and sewed the button on to hold the strip in place.
And, here it is!