Creating a Custom Button/Module

We are attempting to create a custom button / module, and before I put too much more work into it, I am wondering if someone who's had experience creating a custom button / module might be able to let me know if it's even possible or where I might start or what to watch for--any help you can give. Some background: we have a web application that is used primarily by university-level student writers, their writing peers and their teachers. We want to upgrade the manner in which they electronicall ...