The easiest way to make a simple T- Tunic is to lay a loose T-shirt against a folded width of fabric (the wider, the better, with the wide bolts you get longer arms), with the neck against the folded edge and the arms spread out, and cut (don't forget to allow at least an inch for the seam!) out around the shirt. Fold the tunic in half, and mark the center (where the neck would be). Next, unfold the material, and lay it out flat (sort of a cross shape). Cut the material you want to use for the plackard, and hem (roughly a rectangle that is about 12" by 8", depending on prefrence and tunic width). Pin the Plackard so that the long edge of the rectangle is against the fold, and the center is hitting the mark (see below)
Sew down the plackard, and apply trim. If your material wasn't wide enough (or long enough) for the length of the tunic you wanted, sew on extension pieces to the bottoms and arms . I've found that its easier to cut the desired neck shape, and hem or put bias tape around it, while its still flat. Lastly, sew up the sides, and under the arms, hem and voila- a tunic!