To actually build your LibreTexts, you should have a map of your LibreText to be, but just to get started by putting some pages into your sandbox so you can explore the system, we can skip this step, which, as it turns out is going to be the most time and thought consuming.
Before using the OER Remixer, it is important to construct a Remixing Map. This can take many forms and does not need to be constructed online. Lubbock Christian University created an excellent excel sheet to help out in this procedure for their general chemistry course and an image of this excel sheet is shown below. The gist of this map is to identify the content in the libraries (green) that will map into the organization of the new remix (yellow). This requires faculty/instructional designers to explicitly peruse the Bookshelves of Source texts and of other Course Remixes to identify desired content. Once remixed, these pages can be then edited at the sub-page level (e.g., sub-sections, paragraph, pages etc). The Remixing Map reflects only the initial organization of a custom remix. Notice the customized organization; there is no limit other than imagination and the size/makeup of the LibreTexts libraries. While the Remixing Map below does not demonstrate it, it is possible to make placeholder pages in the Remixer if a gap exists in the libraries. New custom content can be added to these pages once the text is created.