There are three steps to building an R package: (Video 1) Initialize the package files. (Videos 2/3) Fill in details for those files. (Video 4) Create a tar.gz build of the package and/or install the package. Optional steps of checking a package, e.g. if submitting the package to a repository, and adding new functions or data to an existing package are featured in videos 5 and 6.
Both a paper describing how to build R packages and slides for an R Package Building Tutorial are available (slides source, license), which includes discussions of some topics not covered in the videos below.