This is a quick tutorial to show you how to make a simple piece of medieval-ish
clothing. The construction method is not strictly period, but the end
result looks good enough for your first few events.

First, obtain your material. Most t-tunics go to just above the knees, but
they can also go right to the floor. The length of the cloth should be just
a bit more than twice the distance from your shoulder to where you want it
to hang to. The width (ideally) will be at least the distance between your
wrists when your arms are held out to your sides. Pick a material that's
easy to work with. Wool is easiest to work with, since you don't have to
treat the edges to keep it from fraying too badly. An old army blanket in
olive drab or deep red (not orange!) is ideal. A grey blanket can be left
as-is or dyed blue.

Alternatively, you can use broadcloth or whatever you find in the remnants
bin. Strong, primary colours are best, with browns and tans less usual.
The concept of "matching" was unheard of in the early Middle Ages, and it
was common to wear red with green and yellow.

Second, fold the cloth in quarters, raw edges together first and selvedges
together second. Note the corner that corresponds with the centre of the
fabric.

Third, take a loose fitting sweatshirt or tee-shirt and fold it in half,
lengthwise. Lay it on the folded fabric with the lengthwise folds together
and the neck at the corner that corresponds with the centre of the cloth.

Mark the outline of the t-tunic using your shirt as a guide. Leave an inch
or more for seam allowance and to allow a bit of looseness in the fit.
Extend the sleeve line out to the edge of the fabric. Choose either a close
fitting sleeve (solid line) or a wide, hanging sleeve (dashed line, usually
for women). Extend the sides into a skirt and mark the bottom in an arc.

Fourth, take your shirt off the cloth and cut where you've marked.

Fifth, make the neck hole. Be careful! Most people don't know how tiny
their neck really is... measure the thing, first.

Open the cloth out by one fold and make a "D" shape centered on the fold and
as big as your neck. Your neck sort-of grows out of the front of your
chest, not straight up between your shoulders. The neck hole must reflect
this or your garb will ride against the front of your neck and be
uncomfortable.

Flip the cloth over and mark a MUCH shallower "D" on the back. Cut this out.

To get it over your head, you need a hole bigger than your neck. Cut a slit
down the front of the tunic from the neck hole just far enough so that you
can get your head through. Cut this by degrees until it's just right.
Since this makes the neck look a bit like a keyhole, it's called a keyhole
neck.

Sixth, sew up the sides and down the sleeves (dotted lines).

Finally, turn the thing right side out and it's ready to wear. Cinch the
waist in with a belt and wear tight fitting pants and you're all dressed to
come to your first event!

This tutorial is from
http://thescholarsgarret.com/northernshores/tunic1.html.