The basic garment is a deep red wool or linen tunic made of 2 rectangles approximately 36"-48" long by 40"-45" wide, sewn together at the sides and shoulders. The body panels are cut across the grain of the fabric, so that there is a selvage (finished) edge at the top and bottom. (Actually, since modern wool is often wider than Roman wool, you will probably have a selvage at either top or bottom.) The body should be quite baggy and loose--it fits like a tent, not a T-shirt! Original tunics were frequently wider than they were long, and could be as large as 60" long by 55" wide.
Short sleeves are an option, c. 6" long by 12" high, cut with the body panels, but sleeveless tunics were much more common. The tunic hangs to the knees or below, but is normally worn bloused over a cord or tied belt to raise the hem above the knee.
The usual neckhole was a simple slit, made by leaving part of the shoulder seam unsewn (about 12"). Many tunics were made with much longer neckslits, so that for heavy work the right arm could be slipped out. This is shown on Trajan's Column and other artwork. To close up this long neckslit so that the tunic will stay on your shoulders, gather a "knot" of slack fabric at the back of the neck and tie a cord or thong around it, or just pin the slit shut with a couple of fibulae (see Cloak). You can also simply put a couple stitches at the two points where it would be pinned. Round necklines (c. 8" wide) were also known, but not common.
Practically any available red wool or linen is acceptable – but get the darkest color red you can find. Blood red is the goal here. Wool fabric need not be blanket-weight, but it should certainly be 100% wool. Linen tunics should be 100% linen and heavy enough that they will survive rough usage.
Undertunics cannot be well documented, but the wearing of one for comfort is an option. Make it of white or natural linen, the same shape as your wool tunic or a little smaller. In very hot weather a linen tunic may be worn instead of wool to avoid dangerous overheating.
Sewing a tunic is very simple. Prewash your fabric! The cut ends of linen will unravel a LOT, so they must be zig-zagged, hemmed, or whipstitched FIRST! Also, allow for at least three inches of shrinkage, length AND width, per yard of fabric. Use hot wash/cold rinse/machine dry for linen, cold wash gentle/cold rinse/line dry for wool. Iron well. Only now should you measure and cut the pieces for your tunic. Here is how the pieces of a sleeved tunic can be fit onto the fabric, depending on the dimensions:
Place the front and back panels together inside out, sew the sides and top, then turn rightside out. Regular cotton thread is acceptable, though it is possible to find linen sewing thread for extra authenticity. We generally machine sew the seams and do the hems by hand, but doing the whole thing by hand is certainly a worthy option. Running stitches (in-and-out) are fine for seams and hems. For hems on linen especially, it is best to turn the edge under twice (very narrow, 1/4" to 3/8") to hide the cut edge completely, then stitch. (Iron these folds down before you stitch, to make hemming much easier.) The cut edges inside a linen tunic should be whip-stitched or machine zig-zagged, either before or after assembly, so that the finished garment can be machine washed when dirty (warm or cold water). Otherwise, and for wool tunics, simply squish into a bucket of cool, soapy water, let sit a while, then rinse and line dry. The Romans used urine (ammonia) and sulfur smoke to bleach white clothing!
A simple tie belt can be made from a long strip of linen or wool about 4 inches wide. Fold the edges in towards the middle, then fold in half lengthwise and stitch.
SOTW is probably the best supplier of off-the-shelf tunics, but making them is more authentic and the colors are generally better. The big danger with an off-the-shelf tunic is that the color will be too bright or off in some other way. If you DO order off-the-shelf, then you need to hand-stitch any exposed seams.