This is what you do.
Cut 2 pieces of knit fabric. Give it a good medium weight, it will be nice and comfy.
This diagram can be used for both the front and the back because the stretch of the fabric. Make sure when you lay out the pattern that the stretch goes ACROSS the body.
If you want to make a modification, I suggest raising the neckline for the back so it does not dip as low as the front (it also will prevent slippage).
Okay, for the cuff, I suggest cutting two rectangular pieces like the picture the length of your arm circumference wherever you want the sleeve to be. Add 1/2" all sides of the rectangle.
Also add 1/2" on all sides of the pattern as well.
How to sew:
Gather along the stitching lines as stated on the diagram. You can mark those with chalk or thread on the wrong side so you know where to gather. The easiest way to gather is to get a piece of clear elastic or narrow elastic, whatever you can find. Measure the gathering line from the top at the shoulder line, to the bottom at the waist. Shorten that measurement by 1" or 1 1/2". Cut the elastic that length. Cut 4x, 2 for the front, 2 for the back.
Along the gathering line, place one end of the elastic so it crosses over into the seam allowance at the shoulder line. Stretch the elastic down to the waist and cross over a little bit into the seam allowance. Gather the fabric to your liking between this distance and sew RIGHT ON TOP OF THE ELASTIC from top to bottom. Keep the elastic on top and the fabric on bottom so it will be easier for your machine to sew. Repeat for front and back sections.
When that is done, sew the shoulder lines on the front and back pieces, RIGHT SIDES TOGETHER. Repeat for the "skirt" section below the waist.
Now, take those sleeve cuffs you cut out. Sew the short side of the rectangle RIGHT SIDES TOGETHER WITH A 1/2" SEAM ALLOWANCE. Fold in 1/2 along the long side of the rectangle and press. Repeat for other cuff. (the width of your cuff is your choice, but when you decide, DOUBLE the measurement and add 1/2 SEAM ALLOWANCE, so you'll be able to fold in half and press).
At the end of the sleeve, run a gathering stitch. Leave the threads long so you can gather loosley or tight, what ever you want. Take the cuff and attach it to the sleeve and adjust the gathers so if fits within the cuff. Sew with a 1/2" seam allowance. Repeat for other sleeve.
At neckline, what I recommend is that you get some binding that you see on the tshirts you wear and sandwich the neckline between the binding an sew along the edge. This will give it a professional look, if not, you can fold over along the 1/2 seam allowance and topstitch.
Repeat for hem at bottom.