You will need...
ModerateFiddly towards the middle. I say fiddly - if it weren't for the one process in question I'd rate it very easy. It helps to know your stuff when it comes to sewing elastic but if you don't, fear not for I will be your guide!
TimeAbout a day, i.e. 6-8 hours. Again, if it weren't for the process with the elastic it'd be significantly less. I don't want to give the ending away but the hole punching part was much quicker than expected.
Total CostThe pliers cost about £12 from Hobbycraft, interfacing generally costs a few pounds, tops. For the t-shirt, you just have to be lucky with charity shops if you don't already have one hitherto in need of a new home - or makeover!
Use your graded setsquare (I used a patternmaster which does the same thing in this tutorial) to mark out 3 lines: a 45° diagonal line about 7cm from under the sleeves and a horizontal line just under the sleeves.
Pin your interfacing to your t-shirt as a reference and trace out 2 triangular pieces from below your diagonal line and a front and back piece from the shoulders to your top line. Cut them out.
Use your grid paper as a reference to trace out some perfectly geometric dotted patterns, as shown in the picture above, by placing it underneath your interfacing. It helps to have a well-lit work area!
Iron your interfacing in place and use your setsquare to measure parallel horizontal lines from 4 and 6cm under the bottom edge.
Now for the tricky part! Cut a length of elastic slightly longer than the circumference of your waist, or as much as you feel comfortable with, bearing in mind you'll need to be able sit down in the dress. Pin it along the top line, stretching it out until the ends meet ans sew it in place with a long, straight stitch along the middle - a tacking stitch which should be easy to unpick. To avoid snags and puckering take your time. If your sewing machine "eats" your fabric at all, simply unpick the stitches with a seam ripper and go over it. Use a small zigzag stitch across the top and bottom of the elastic, again, holding it under tension and stretched out, then unpick the tack stitching.
Use you eyelet puncher to create grid-like perforations through all your marked dots.
I tucked in the sleeves, fixed them along the armhole edges with a single row of stitches and cut away the excess fabric but that part's optional.