1. Put R code in 'R' subdirectory.
2. Put C/C++/Fortran code in 'src' subdirectory.
3. Add .First.lib() function in 'R' subdirectory to load the shared library.
4. Edit the help document in 'man' subdirectory.
5. Edit the demo function in 'demo' subdirectory.
6. Run R CMD build to create the index files.
7. Run R CMD check to check the package.
8. Run R CMD build to make the package file.

Read "Writing R Extensions" for more information.
And get more information from,
SHELL> R CMD check --help
SHELL> R CMD build --help