cut off both sleaves and then down the centre of the back of the t shirt

then make 11 diagonal cuts from the centre to about 3" into the front, depending on size of t shirt and of you... i streched mine around me and marked my sides as a guide.
Then cut off the excess triangles at the to leaving the side edges to form the straps.

cut a neck line into the t shirt, be conservative to start with you can always trim it again when it's finished.

grab a pillow to make it easier to weave, pull each strand individually to strech it to it's full length. weave the strands together, knotting the 1st strand to the base of the 5th knot it into position (but not too tight for ease of fudgeability later) leave the last 5 strands free to knot onto the straps.

cut off the excess to make a wide strap.

knot base of strap tightly and as flat as possible.

tie the loose ends to the strap as tightly as you can, you may want to secure with stitching. sew buttons to the top strands to keep them in the correct position as pictured. try on and fudge where needed and chop off any excess.
Now all thats left to do is look in the mirror and say "Ta Da!"