A tunic is the most basic of all clothing designs, as well as the most popular. It is versitile, and can be nearly any length, though it almost always reaches below the waist. It is always made of one fabric, but that fabric may be dyed or embroidered in any manner of ways, and the neckline and style of sleeves may differ greatly. Tunics may be belted, as well, for a more fitted look.
There are four basic parts to a tunic. The body, the sleeves, the gores, and the gusset. The gores are triangular pieces of fabric that are sewn into the skirt of the tunic to allow movement. Imagine it like having just a few hidden pleats in the tunic. The gusset does the same thing, only it's square pieces that fit under the arm pieces. The key is always to know how much movement your subject will require. The more movement the more width you give to these items.
To calculate how much fabric you will need, take the length you wish your tunic to be and double it.
Measure the subject, including head, neck, chest, tummy, neck to wrist, arm circumference, waist to hem, shoulder to hem, etc. These will let you know how much fabric will be needed to comfortably cover the subject. When cutting your pieces, make sure they will allow for these measurements, as well as a little extra to allow for seams. Trim is best added before the entire tunic is finished. This is also a guideline for all clothes constructed. Other pieces that are not as simple may be created by following stocked patterns, which one may trace onto fabric, pin together, and then sew.
The majority of information here was taken from http://www.virtue.to/articles/tunic_worksheet.html
This website goes into far more detail, and provides illustrations.
