We started out with Bethany's big shirt. She had brought another that fit well, so we used that to trace a pattern on the big one (with both shirts inside-out, so we weren't drawing on the outside of the shirt!). We tried using a fabric pencil to do the tracing, but it was way too light to really see, so we ended up using a gray washable marker. I don't think it really matters, though, because the line ends up inside the seam.
Once we had the pattern drawn, we got to work sewing. Bethany hadn't used a sewing machine in a long time, so I showed her how, then let her go to town. It was her shirt, after all, I figured she should do some of the work.
Here is one part that wasn't mentioned in the tutorial I found online. When you're doing the tracing, you may want to shorten the sleeves. Go ahead and trace that part out, but DON'T sew along that line. If you do, you will sew your sleeve shut. We did that on the first shirt we tried, and there was seam-ripping involved. On the plus side, I finally went out and invested in a seam-ripper (or rather, Bethany went out and got one for me; thanks, sis!). Instead, follow the lower line of the sleeve and sew out to the edge. After you cut off the excess, hem up the sleeve the regular way you do a hem.
We did 1/2" hems; folded up, ironed, folded over again, ironed, sewed. We also hemmed up the bottom to shorten up the whole shirt.
The other thing we did that was not mentioned in the tutorial was to zig-zag stitch the raw edge where we trimmed off the excess fabric. That makes that edge inside the shirt a little nicer and will help keep it from fraying. I did the zig-zag stitching before doing the sleeve and bottom hems. Now Bethany has a shirt that fits much more appropriately than before. And she got a crash refresher course in using a sewing machine, which is always a useful skill.