We present a first prototype of the tool SentiAnno, which is an annotation tool specifically designed for sentiment and emotion annotation in structured written texts. To design this tool, we employ the user-centered design (UCD) framework and adapt it by focusing on the annotator, thus phrasing our development approach annotator-centered design. In iterative steps, we gather requirements and feedback from annotators as soon as possible in the development process via various usability engineering methods. We propose that this design process is especially beneficial for challenging and subjective annotation tasks like sentiment and emotion annotation of literary texts. We describe our first iterations and present results of the current prototype. We show how we were able to create functionalities facilitating the annotation process by applying this annotator-centered design approach.