I started out with an existing tunic. But, since
most in search of a tunic currently do not have one, then use a
loose fitting shirt as your basic pattern. My tunic looks similar
to a mid-sleeve t-shirt jersey with a v-neck (with adjustments).
I use only four pieces -- one for the front, one for the back,
and two small pieces for the inside of the neckline (one short
and curved for the back, one pointed to match the v-neck or other
style on the front of the tunic).
Since my pattern is an existing shirt, I do not fold the material when cutting. I open the material, which should be 60" wide, to make one layer. I lay the shirt flat on top of the material and cut around it, making any adjustments (under arms, length, collar, etc.), and, of course, leaving an extra 1-2 inches for seam allowance (2 for the bottom and sleeve hems). I typically use my left index finger to measure, while I cut with my right hand. (I'm real precise about these things!) For the front, as I have said, I typically use a v-neckline. Once I cut out the front and the back, I use the left over scraps to make the inner neckline. Be sure to cut, as always, with the selvages (finished edge of material) or what would be the selvages on the sides of the pattern, so the material is laying the appropriate way. The inner neckline pieces are about 2-3 inches wide, and basically, you lay some material over the front half of the shirt and cut along the neckline and shoulder matching the tunic edges, then cut the piece making it about 2-3 inches wide. Do the same for the back.
Next, I sew the front and back together at the shoulders, with the right sides of the material facing each other. I sew the pieces of the inner neckline together at the shoulders and use an edge stitch (zig zag) on the raw edge of the inner neckline. Pin the inner neckline to the tunic neckline, right sides together, matching shoulder seams. Sew around the neckline, trim. Flip the inner neckline to the inside of the tunic, iron. Then, with right sides together, sew the sleeves and sides of the tunic. If you wish to leave a slit on the lower edge of the sides, stop and backstitch at the desired distance. To reinforce the underarm, sew 1/4" from the seam on the raw edge side for the curve of the underarm. Hem the bottom. It helps to measure the fold for the hem and iron it before you sew, but if you prefer to wing it, feel free. Either edge stitch the raw edge, then fold desired hem and sew near raw edge or fold desired hem and fold the raw edge under 1/4", then sew within the 1/4" fold. Do the same for the sleeves. If you left a slit on the sides, fold the seam allowance down to corresponding half of tunic, and sew around the slit (similar to a thin, long rectangle with an open end.) On the short end of the rectangle (where the slit meets the side seam),backstitch the area and stitch again to reinforce the slit. Now you can add the trim. It is easiest to pick a trim that is flat, not three dimensional. Simply pin the trim where you wish it to go, usually around the neckline, sleeves and perhaps the bottom, folding the trim at any angles and tucking ends under, letting them overlap. Sew trim in place on both sides of the trim, so it will lay flat.
It took me about 4 hours or so to complete the last tunic I did. As for length of material needed, you will need the length of the tunic plus hem and seam allowance times two, perhaps a little extra for the inner neckline, but I can usually make those from the scraps that are left over. I believe I did the last tunic with 2-3 yards of material. If you want to practice first, I suggest using some cheap material (like $1.00/yard at Wal-Mart, preferably not stretchy material; it can be difficult to sew), or perhaps broadcloth, though it may be hard to find 60" wide. As always with "period" or ritual garb, 100% cotton is the best! It's cooler and all natural.
Good luck on your sewing adventure! And fare well with the Gods and Goddesses!
For those of you interested, here is an SCA link with instructions on making various kinds of tunics: http://www.cs.vassar.edu/~capriest/viktunic.html