This is my first tuto ever! I've published some T-shirt dresses before, but this one is my own creation, out of two contrasting boyfriend tees! If you have any doubt (remember English is not my mother tongue!) please ask in the comments ;-)
Start with two old, oversized tees in contrasting colors.
Take your underbust, waist and hips measurements, as well as vertical measurements. Divide the horizontal measurements by 2 (half will be in the front, half in the back).
Choose the main color t-shirt (mine is yellow) and lay it inside out on the table. Remove the sleeves (we're using them later) and cut straight across the top of the t-shirt.
Draw the middle line in front and transfer your half-measures over it, MINUS 3cm each side. So, if you have a 64cm waist, you are going to draw half of it - 32cm - minus 6cm - or a total 26cm. Same applies for underbust and hips.
The underbust line shall be put over the cut line at the top. For the hem line, I just draw a diagonal line from the hip measurement to the corners of the t-shirt.
Then draw the general dress form using your measures and pin the front and the back of the tee together.
(another explanation of the drawing method can be found here: http://www.cutoutandkeep.net/projects/t-shirt-dress-3. Note that we are changing the measurements since we're using 2 colors)
Cut out the shape over the front and the back of the tee at the same time, letting 1-2 cm for sewing.
Now for the black gusset:
Pin the yellow form to the wrong side of the other t-shirt, in about the same position so the hems match. Draw around the form with your chalk.
Prolongate the horizontal lines and include 3cm each side, plus 1 or 2 cm for ease (or even more if you prefer a larger fit). Then re-draw the gusset form each side. On the next step I will show how I did my flared skirt...
I don't like a dress too snug over my hips, so I created a triangular gusset on the lower part of the black t-shirt. Lift the yellow t-shirt a bit over the hips (you can see the hip line in the pic) and from there draw a triangle to the inside of the black t-shirt.
Pin the gussets and cut, then sew the outer side closed.
Now for the top part: since I'm quite slender and my tees were XL size, I could do it only with the black tee sleeves, but fuller figures will maybe need some more material. There will be a lot we haven't used from the black t-shirt, or you can sew together both black sleeves for the front and both yellow ones for the back, or invent another solution which I haven't thought of!
I'll explain the method used for my dress and I hope you will show me your solutions in the comments!
Cut open both sleeves and mark the middle line of one of them. For the front part, cut a 18cm width band out of one of the sleeves, keeping the hem at the top; for the back part, the band will be 12cm (just enough to cover my bra strap!).
Measure the length of your bands, cutting them up to the measurement of your bust (my suggestion: pin one of the sides and "wear" the long band over your bust, aligning the pinned part with the side of your body. Pinch the band closed on the other side, pin/mark and cut, letting some sewing space)