This is how I resize t-shirts, sweaters, cardigans etc... it works for everything.
First take the garment you want to resize and lay it flat. Make sure that there are no wrinkles on the bottom side of the fabric. This sweater is my husbands and was just a bit to baggy all around.
Then take another sweater that fits properly and lay it out on top of the first sweater.
Trace along the edge of the sleeves and mark the difference with some tailors chalk (I like to use this form of chalk, it just make me feel more 'authentic' i don't know how else to put it)
Then pin the fabric off to one side of the chalk line, so that the fabric will stay in place and not move around when you sew it.
Sew on the chalk line.
Fold in half to make sure both sides are even.
Cut off the extra fabric. Turn inside out and then you are done!