Here's the technique I use for making a circular neckhole:
1) Fold the tabard in half lengthwise.
2) Fold it again widthwise.
3) Find the corner that's where your neck should be. It'll be a solid point with no edges of the fabric visible.
4) Using an bowl about 6" in diameter, lay it upside-down on the fabric and position the center of the bowl approximately at the point from step 4.
5) Using chalk, draw an outline of the bowl edge. It should look like a quarter of a circle.
6) Cut the fabric along that line. Scissors work but I've found that a rotary knife works better for me.
7) Unfold the tabard and you should have an almost perfect circle.
8 ) After that, figure out which side is the front. You'll need to cut a slit down the middle of the front long enough to fit your head through. (aka keyhole)
9) If you make your circle large enough, you won't have to do a slit in the front.
As seen in the link below, you can also do triangles and squares as neck holes if you're looking for something with straight lines instead.
http://greycompany.com.au/about/costumes/tunic-and-pants/Hope this helps.
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