For the steps required to get a boot-able SD please follow this video:
For more resources about DE10-Nano :
Note that the previous link has a ready to go applications for your first FPGA or HPS projects which will help you to create the required project.
I see the RTEMS is based on Free RTOS so this link will be helpful for you.
You can also go to RocketBoards.org https://rocketboards.org/foswiki/Main/GettingStarted
Then select the board and the Tool version and the task you want to do
then you will get a full description of how to do this task.