For my part Nihilius was particularly taxing, being that it was my first costume, the hardest part I found working out the hood and horns to look right... The inner tunic was not much better. At any rate, when it's all said and done it comes down to the opinions of your peers, so exactly what is right and what is wrong leave allot of room for interpretation.
I don't recall what pattern I used for my inner tunic, how ever for the sleeves, I simply laid out two sheets of fabric and sewed some batting onto one side, then attached the other sheet and adding the lines of stitching at regular intervals for the quilting effect, as for the shoulders well one trick we used was to measure the length of my arm while it was bent at the elbow, and I added some stirrups made from elastic in the ends to keep the sleeves from riding up.
Personally I found open front tunics for the inner and outer tunics to be the best bet, much easier to slide on and off, adding some ties will help keep the tunic closed, perhaps some Velcro as well. The obi will keep things closed as well. The hood and horns are the toughest part I found. I used an old East German hat to hold my horns and secure my tunic, I want to refine the hood but the build I have now feels more durable. I used the flaps on the sides of the hat to tuck the hood in to keep it close to my face, and my mask actually sits over the outside of the hat with the strap over the flaps. The hat keeps the hood in place and well tucked behind the mask, so the mask is actually free from the hood.
As to my belt, well I used two pieces of leather, and placed black rivets along the middle of the center leather strap. With snaps in the back to keep it tight, the back is buried under my capes so no one will ever see it. The skirts were the easiest part, I just took to looooong lengths of cloth and placed a tube in the top to house and elastic cord.