I took photos while I did it and put together a little tutorial on resizing, just because :] and who knows, I might just help someone.
Started off as a size L - brand new, never worn, stashed away in my brother's closet for years. :o
Ended up as:
(woo it's David Grohl in the background!)
This is for bigger, shapeless shirts that can't just be taken in without having funny looking sleeves. It takes longer, but it looks more professional.
I don't recommend it if you're not too keen on cutting up your shirt and trusting some random stranger.
You'll need: a t-shirt that fits, a t-shirt that doesn't, a measuring tape, some pins and thread, something to draw on the shirt with (something that will wash out - I use dry pastel pencils cause that's what I've got) and a sewing machine!
If your big shirt isn't stretchy, try to use a t-shirt that isn't stretchy for the template. Otherwise, you might not be able to get the top on, especially over your boobs (it's happened to me before... :P woo)
I apologise if my photos aren't very clear. My camera is old and crap. :P
Turn the shirts inside out and put them on a flat surface. Floor works. Carpet works best as you can stick pins into it.
Put the shirt that fits on top of the other one, and make sure there are no creases or anything. Align the shoulders and neck and trace just the sides, from below the armpit. Make it as long as you want. (forgot a photo :x)
Now, pin all around the arm hole. As I do this on my carpet, I just stick the pins in so they don't move around. Lift the shirt up so you can see where the pins meet the big shirt underneath and mark where they are. Just dots or little lines around both sides of each pin will do - this is so that you get the armhole traced properly, as curves are rather tricky.
Connect the dots! This should give you a nice curve.
Here you may wish to make sure both sides are symmetrical. I didn't take photos of this step, but I folded my shirt in half, ironed it so I got a vertical crease down the middle, and measured how far the lines on each side are from the crease. I usually skip this, but this shirt had a design in the centre, and I didn't want it crooked.
Now you've got the body of the shirt traced on the front. To get the back identical to the front, pin the front and back together. Make sure the pin enters and leaves the shirt on the lines you've drawn.
When you flip the shirt over, you'll see the pins are exactly where the lines on the other side are. Mark where the pins entered and exited the fabric, remove them and connect the dots.
Do this to both armholes and the body of the shirt.
Add seam allowances. I do this by putting one side of my tape measurer to the line and marking the other side. Mine is 2.5 cm wide, which is just right for me.
(You don't have to do this my way! Just you leave plenty of space, in case you have need a bit of space to make the shirt bigger. Also, my sewing machine is moody and eats up thin fabric if it's not being held down properly by the foot presser :P)
Sew up the sides, loosely by hand, only and try it on. If it doesn't look okay, adjust it. If it looks fine, cut it!
Now you should have two sleeves left. Grab one of them and put it flat on your surface. Take your fitting shirt and put its sleeve on top of the other one. Make sure the hemmed sides are aligned. Repeat steps 2 to 5. Repeat for the other sleeve.
This is what the sleeve looks like compared to the armhole (the shape is supposed to be different).
Pin and sew! Do the sleeves first, then the sides, and finally hem up the bottom.
To do the sleeves, pin them at the top first and work your way down to the armpit, on both sleeves. If you've been careful and accurate, it should fit perfectly. If it doesn't, try to at least make sure it's equal on both sides.
To do the sides, pin both sides together from the arm, to the armpit (follow your lines) down to the bottom.
Hem up the bottom, and you're done!
I hope this wasn't a bit redundant of me or anything! I think photos are the best to understand and well. If it sucks at least I'll provide a laugh or two. :D