You'll need a buddy to trace the shape of the robe around your body.
For a billowing robe, trace farther from the body. For a robe that has a better fit, trace 5" or so away from the body.
For the sleeves, start 8" from the top of the fabric for a close fit. Trace 14" or more for long, draping sleeves.
Once you have made your marks, go back and adjust your lines to make sure the measurements are even. If one sleeve is 18", make sure both sleeves are 18" and so on.
Next, we will cut out the robe.