turn your oversized t-shirt inside out. Turn your fitting t-shirt inside out, tucking the sleeves in. match up the necklines and pin the fitting t-shirt onto your oversized t-shirt like so
cut around the fitting t-shirt, adding a seam allowance. Sew up the side seams. Sew some lace around the neck if you like.
cut the sleeves off the remains of your oversized t-shirt, as close to the shoulder seams as you can. Pin and gather to the armholes on the base you just made. Sew, and add some lace to the edges if you desire.
cut two strips of elastic the same width (or a little bigger for comfort) as you upper arm. Sew the ends so you end up s you end up with two loops
stretch and sew the elastic to the ends of your sleeves to gather them. You now have the main part of the cutsew
lay your cutsew on your long sleeved t-shirt, matching up the shoulder seams. Cut off the sleeves a bit above where the ends of the sleeves of your cutsew comes. Put aside your now not so long sleeved t-shirt to fix later.
repeat step 4
zig zag stitch or serge the unfinished edges of your detachable sleeves. Stretch and sew the elastic on to these edges. Add lace to the wrists if you wish
sew two hooks onto each of your detachable sleeves. Sew eyes to the corresponding positions on your cutsew's sleeves.
Now take your blouse. I'm using one which I adapted ages ago, but didn't wear because the sleeves where awkward and there was paint on the hem. It used to be a plain school blouse with a shirt collar. To make a collar like this, simply unpick the seam on the edge, cut the ends off to make it round, add lace and ruffles and resew.
cut down the arm seams, across the back and front so you end up with a shape like this
open the shoulder seams like so:
Make some pintucks down the front, or sew on lace trim if you'd prefer.
cut down the back so it's now the same size as the front. Reattach the shoulder seams. Zig zag stitch or serge around the raw edges.
gather some eyelet lace around the surged/zig zaged edge to make it pretty ^^
remove the buttons. Attach some poppers instead, making sure the old button holes are on the bottom.
sew some cute buttons where the old ones used to be
And then it's done! You can wear your new cutsew in several ways, and the collar can also be used on other tops, or even worn under a JSK when it's too hot for a blouse.