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. In essence it is a table of contents for your remix and a good place to start would be the ToC of a textbook that you like.
Remixing maps are best assembled in a spreadsheet, for example
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 or new open material brought into LibreTexts.
While it is easy to remix content that already is on LibreTexts, the LibreTexts team will bring in open material for you, although this may take some time, depending on the format of the materials and the amount of work that has to be done for such things as equations, images, accessibility issues, etc. Priority is given to materials for institutions that are members of the Libretexts Network. Requests can be made at https://harvest.libretexts.org.