Start by laying out the t-shirt front side up and cutting 2 inches in from each side, to the arm pit (see the photo).
Then cut around the back of the sleeves and all the way across the back.
Your piece of fabric should have this sort of shape on either side, so it's narrower at the top.
(I'm using a mannequin but I'll try and make it non-mannequin friendly too)
Now, because I'm skinny, half a t-shirt will go around me fine but I realize there are people with healthy build bodies out there so I'll talk about that in the next bit n_n
Here you want to measure around your rib cage and then measure around the t-shirt. Any difference will be made up in an insert but if it's the right sort of size then go ahead and pin it for about 3 inches.
You'll want to measure the gap in the back of the dress and cut the right sized triangle (otherwise your bum will be on show). I folded the front of the shirt and cut it that way so it was symmetrical.
Now just pin your insert shape to the gap in the back of your dress and sew it together.
Ta da! Your bum is now safe from the eyes of strangers n_n
Here I have marked my neckline and followed it around to the back of the dress.
Once you know where your neckline is you can cut it out! Then you go ahead and pin it and sew it.
In the picture you can see where I have cut a little slit in the V of the neck. This helps to make a nice sharp V-shaped neckline when you're folding the hem n_n
Now for the straps! I measured mine as needed to be about 19 inches for the shoulder ones and about 11 inches for the one across the back.
So, cut 2 strips of t-shirt 20 inches long and 2 inches wide. Cut another strip 13 inches long and 2 inches wide.
Now we fold it in half and sew it quite close to the edge.
I couldn't really photograph this bit, but, you now want to iron the strap so that the seam you just stitched is down the center.
Now use the zig-zag stitch to secure that seam, it will also strengthen the strap a bit.
Repeat this for all three straps.
Now to attach your straps! Just pin them to the front like so...
... then pull them over the shoulders, crossing them over, and pin them where you want them.
Take your third and shortest strap and pin it horizontally across the back of the dress.
If you want you could add more straps. Go crazy!
You now have your very own reconstructed T-shirt dress... or top depending upon how daring you are.