WARNING: I've only been studying ESL for two years, so there WILL be some grammatical mistakes. I apologize in advance.
I converted a Star Trek shirt a few weeks ago, and when I saw this Batman shirt on the men's department I couldn't help myself. Since big baggy shirts aren't my style, I turned it into a more feminine top, just as I had done before, taking one I already own as a model.
So here's what I did.
Get a shirt! I know it's not that big, but still, it's not what I'm used to.
Turn inside out the shirt you'll work with and one you own and fits you well.
Mark the outline, and when you're satisfied, baste stitch (or join with pins)
Turn inside-out again and see it fits. Yes, it looks funny and the sleeves are all kinds of wrong, but we'll fix that.
Now just make sure it is symmetrical and not too big or small.
Back to the inside of the shirt, we'll get rid of the sleeves. For me, the space between the neck and the sleeves was too long, so I marked where the sleeves in my other shirt started.
And cut. I usually leave a 1 cm seam allowance. If you need more and/or can leave more, do it. Just in case.
Sew the sides.
The body of the of the shirt is almost ready, and since we'll leave the hem for the end, we'll make the sleeves now.
I am personally fond of slightly puffy sleeves, so that's the kind I did.
If you don't like them, you can always just ignore these steps and make the sleeve smaller, or as you wish.
Baste stitch the top of the sleeve. I stitched about 9 cm. Pull the thread to create a 4 cm smoking. Make sure the sleeve is not too small.
Cut the sleeve about 14 cm from the top, and repeat the process on the bottom.
Sew the sleeve to the shirt.
Remember that part we cut from the sleeve? Well, I did this.
And then I did this. And then I topstitched it.
If you're satisfied with the result, repeat the process on the other sleeve.
Otherwise, review and fix any imperfection.
Now the lower hem.
Take your model shirt and mark the hem. Cut about 5 cm below your mark.
Fold it twice and topstitch.
And you're done.
Congratulations! You've successfully made a t-shirt smaller, and revamped it with puffy sleeves.