When it comes to making your own costumes, you’re going to find that more then likely there isn’t going to be a perfect pattern for what your want to make. Most basic shapes (like a pleated skirt, or a basic blouse) you can easily find. But a lot of clothing from different anime, mangas, videogames are pretty strange. So how do you end up making it? If you can’t find something, the only way you’re going to be able to make it is by modifying a preexisting pattern.
One way to modify a pattern is to add another pattern piece to it. Basically you have a base pattern that might have the shape you want, but is missing an element you’d need, while another pattern has that element you need.
One particular example of how I modified a pattern was for Iroha and Meiko. Both of these costumes use the same pattern base for the top, Simplicity 2341, but they look fairly different in the end result. Most of the alterations I did between the two were in the sleeves. The original pattern is just a singular long sleeve. I use that long sleeve as the base pattern to create two different sleeve patterns. For Meiko I needed something big at the end similar to a kimono sleeve. While with Iroha She had a double sleeves, a short layered with a long sleeve. The way I made those was to cut the sleeve pattern, after I cut out the long sleeve, into a short sleeve and used that.
Another example of how I’ve modified a pattern is my Fuu costume. The original pattern I used was Simplicity 3618, which was a costume pattern. I used the pirate costume as the base, because the vest had the underbust that I needed for the costume, but there was some changes I needed to make to fit my reference. I wanted it to be all one piece, so when I was patterning out the costume, I cut the vest pattern about at my natural waist and attached the skirt to it.