Building an RMLinux system is not a simple task. It requires a certain level of existing knowledge of Unix system administration in order to resolve problems and correctly execute the commands listed. In particular, as an absolute minimum, you should already know how to use the command line (shell) to copy or move files and directories, list directory and file contents, and change the current directory. It is also expected that you know how to use and install Linux software.
Because the RMLinux book assumes at least this basic level of skill, various support forums are unlikely to provide you with much assistance in these areas. You will find that your questions regarding such basic knowledge will likely go unanswered (or you will simply be referred to the RMLinux essential pre-reading list).
Before building an RMLinux system, please consider familiarizing yourself with the following articles:
Software-Building-HOWTO https://tldp.org/HOWTO/Software-Building-HOWTO.html
This is a comprehensive guide to building and installing “generic” Unix software packages under Linux. Although it was written some time ago, it still provides a good summary of the basic techniques used to build and install software.
Beginner's Guide to Installing from Source https://moi.vonos.net/linux/beginners-installing-from-source/
This guide provides a good summary of the basic skills and techniques needed to build software from source code.