this is another shirt i made out of a huge t-shirt. i just kinda winged it so i hope i don't confuse ya to bad. my pictures are kinda bad because i have to take them on myself in the mirror and my camra hates that!
now cut this out from the front. i cut out the top seam also you dont want the shoulder seam since were adding straps to the back.
then you will want to trim off a bit so that it gives it a nice v in the front and leaves room for the side panels. to do this i just took a ruler and went from the other corner of the sleeve area and traced a line down about 4 in from the bottom outer edge. i would trace both make sure there even and then cut. you know for sanitys sake.
now for your side panels they should be the width of your side and the length you measured for your back piece and then just cut a triangle. its easier to draw out your waist length on the bottom and then find the middle and draw a state line up from the middle the height of your side measurement and use a ruler to connect the top and sides.
now to put it together. this is the kinda tricky part. you can do these side panel lacing two separate ways you can put in some d-ring trim or you can sew the ribbon directly into the panels so that it wont move or maybe you could sew some hoop rings in well that 3 ways i had some d-ring tape so i just sandwiched my pieces in right sides together with the d-ring trim in the center and sewed.
to sew the ribbon in the panels it you have to fold the ribbon in and pin it while you sew. i'm not even going to try to explain this i saw it somewhere and trying to reexplain will only give us both a headache!
but this is how your pieces should look when you have then sewn in and then sew in the back panel making sure you line up your d-rings.
repeat for other side.
then i went and hemmed around all my edges top and bottom.
then for your straps in the back i just tried my shirt on and measured from the top of my shoulder to where the back came up and cut out two rectangles. i hemmed up the long sides then pinning them to the back and sewing then on then sewing the shoulder pieces together.