This is what the bodice pieces for a normal t-shirt will look like. As usual, it's much easier to do any pattern alterations without seam allowance.
Before you do anything about the twist, the basic shape of this top has to be made. To do this, you raise the shoulder point by 1 cm and extend the line from neck to shoulder by about 12.5 cm (to decide how much, put the normal t-shirt on and measure down your arm from the shoulder point. In my tops, the sleeve which attaches the seam is at the bicep). From that new off-shoulder 'shoulder' point, draw a line down to the body, at a 90 degree angle, and merge it into the side seam.
Shape the new neckline. The basic pattern in this drawing has a close fitting neckline, the top we're making has has a wide V. Widen the neckline so that only 7 cm of the original shoulder line is left. At the back, the neckline needs to be lowered by about half the amount it was widened. At the front, make a V which ends at the bust line.
If you don't intend to finish the neckline with a band, you definitely have to make gape darts at this stage too. I'd say 1.5.
Then there's the sleeve to think about. This will be a very simple shape. I'll give you the measurement I used for mine but I know I have fairly skinny arms, so only use these as a starting point. You want to make a sleeve which is a bit loose at the top and tapers to a closer fit at the wrist. The sleeve will be a bit wider at its top than the original sleeve-with-sleevehead was at that that point.
Draw the line for the top. Mine is 30 cm (the circumference of the middle of my upper arm is 27). Draw a line from the middle of this one, straight down. The length of this one should be the desired sleeve length (which you can measure on a straight arm). With the bottom of that line as a center point, draw the bottom of the sleeve, for which you can use your wrist measurement. Draw straight lines for the sides. I like to make a very shallow sleeve head by making the top of the sleeve 1.5 cm higher and drawing a smooth curve from it to the sides.
And now it is time to start the twist! For the shape of this top it is important that the twist itself sits at bust level and there is no extra width added below the waistline.
So, draw in the lines with that in mind. The actual seam will line up with the sleeve so it will end at 15 cm below the shoulder point.
All the blue lines in the picture are where you will slash the pattern to form the twist. The red circle has a diameter of about 5 cm and is there only to help you add the extra width which the twist will need (which is easier to work with when drafting the pattern but really tricky to get into this drawing...)
I usually slash the pieces above and below the seam into 5 pieces, connected through that circle. In practice, this is a point where it's easy to go wrong. To avoid confusion after cutting, I like to number the corresponding pieces from the seamline out: So the pieces right above and below the seam are 1, the next ones are 2 etc. This may sound very obvious but as soon as these pieces are cut, it will become very easy to get them confused. This is also a good way to make extra-sure you've made the same number of pieces in both parts. Keep in mind that that lower body piece also counts!
And after all those warnings, it's time to slash and spread (you will see that I've suddenly gone from 5 pieces to 4. Please ignore that. I had made a mistake while drawing these and I didn't have an intermediate stage saved. And I didn't want to start all over again. For the principle of the twist, it doesn't matter).
Start with the largest piece, the lower body and match each next piece at the outer edge. Add about 5 cm between the tips. Mirror this with the upper parts, with the '1' pieces meeting at the tips and the '5' pieces having diameter of the circle, about 5 cm, between them (the drawing is wrong about this as well, my apologies). If you had any darts (bust darts or gape darts) fold them closed at this stage.
Trace the pieces and smooth the edges to draw the new outline of your pattern. Don't forget to make notches for the twist.
The neckband is a very simple thing: measure the neckline at front and back and subtract 10%. Draw a straight band of that measurement and 6 cm high and take out a piece at one end at 45 degrees (or more or less, if your neckline is far from a 45 degree angle to center front).
Now, add seam and hem allowances to all pieces (if you prefer working with pattern with seam allowance). The neckband will be cut with the short straight side on the fold, the sleeve has its center line along the straight grain, the center back of the top is cut on the fold. The front should actually have the lower center front on the straight grain but if you're working with something like a stripe, it's often much more fun to put the neckline or one of the edges at the twist along the stripes.
I just added this image to clarify things.
When sewing, you start with the twist: If necessary, finish the raw edge of the fabric between the twist notches.
Sew the twist seam on one of the front pieces from side to notches. Pull the other front piece through the remaining hole until its twist notches are on either side of it. Make sure none of the pieces are twisted back to front.
Sew the twist seam on the second piece and the center front seam.
Sew the shoulders and attach the sleeves with their tops at the shoulder seam.
Sew the side seams and sleeve seams in one go.
Now the main construction on your top is finished. For the neckband, start by sewing the pointed bit together as it was pinned when you cut the pattern piece. Clip the seam allowance to the point, turn right side out and press flat. Now you've got your V-shaped neckband.
Pin it to the neckline, carefully distributing the excess ease (that 10%...). The twist notches should end up at about 1.5 cm from the tip of the neckband seam allowance. Starting at the tip, sew on the neck band.
I usually do the entire construction of this top, apart from the center front seam of the neckband (I sew that using a very short straight stitch), on the overlocker/serger. But, if it's your first time making a design like this, I would definitely recommend sewing the seams around the twist first in a way which gives you a lot of control and will allow for easy unpicking.
If you don't have a serger, you can use all the same stretch stitches on a the sewing machine which you would use for any other project using stretchy knits.
I hope this is clear to you. As usual, feel free to ask questions and I'd love to hear from you if you make something using this tutorial.