This tutorial is specified towards women as I am a girl but you might be able to follow the basic steps for men.
I don't know how big or small you'll want your button up shirt to be. As you can see in the picture the shirt is enormous on me (that being why I grabbed it from the thrift store). Take in mind, however, that I did bring in the sides a bit because it was just a little too large.
The pictures may not line up exactly with the steps because I've moved the steps from my original plan as my original plan wasn't working.
I highly suggest you read and understand all the steps before working on your shirt.
Let me know if you have any problems or questions.
Remove the pocket if your shirt has one.
Sew the button part closed. (See the picture for a better idea of what I mean.)
I'm using a zipper foot in the picture.
This part can be a bit tricky as it's not precise.
Cut the neck open (keeping in mind where the shirt lays on you and where you want the neckline to be when you're finished).
I suggest cutting the opening smaller than you want it, try it on, and continue until you get it just about right.
Also you're going to want to cut it bigger than where you want it to lay in the end. We're going to put elastic here and that will also draw it in and up.
Fold over the neck opening twice; creating a casing for the elastic.
Measure yourself where you would like your shirt to lay.
Cut a piece of elastic this length.
Pin the elastic to the shirt and put a safety pin on the other side.
Feed the elastic through the shirt.
I suggest pining the elastic to the shirt once it is feed through entirely and then try on the shirt. Just to ensure the elastic is tight enough.
Sew the two ends of the elastic together and sew the casing close.
After cutting the opening to where you want put on you shirt again.
On both sides place a pin at your armpit, your bust and your hips.
Lay your shirt flat and decide if your bust or hips are wider.
Remove the pin of the less wider one.
Figure out how much extra moving room you want. I went for 2 1/2" (on each side)
Mark that and the armpit with chalk, tailor's pencils or whatever you like.
Draw a line that goes from the armpit to the hips.
Don't draw a straight line down from the armpit. The point is to connect the widest part to the armpit. So it will end up more diagonal than straight.
Does that make sense?
Sew this line. DO NOT CUT OFF THE EXCESS FABRIC.
Try on the shirt. For me it was too small in the bust and too high in the armpits. Adjust as necessary.
Cut off the sleeves (and the excess fabric if you please).
Remove the cuff and other fixings from the sleeve.
You'll probably have this gap in the sleeve. Sew it closed.
Remove extra, unnecessary pieces of fabric from the top of the sleeve.
Use a running stitch on the top of the sleeve and gather it.
Pin the sleeve to the shirt and sew it.
I don't have a picture of this step so just enjoy this picture of my cat no longer wishing me to sew this shirt.
Decide where you want the sleeves to come. I wanted mine to come just above the elbow.
Leave extra fabric to make a casing for elastic.
Prepare the same as the neck in steps 4 - 6
If you would like, cut the length of your shirt shorter and hem.
Dye your shirt in a tea bath. This will give it a more aged/worn look.