One of the benefits of using an openly licensed textbook or other educational resource is that you are free to adapt it to your needs. In other words, you can adjust the educational resources to fit your course curriculum, not the other way around. So the first step is to locate an open textbook or other OER suitable for your course, and then decide what changes — if any — you’d like to make.
Before adapting an existing open textbook, read 6 Steps to Modifying an Open Textbook. This page outlines the basic workflow of an adaptation.