Your starting point is a working R:TW installation.
The suitability/availability of mods that you might want to install depends on the version of R:TW that you own.
It's often best to create a copy of your working R:TW installation folder and install the mod on top of the copy. This involves running a downloaded installation program or maybe extracting a downloaded archive into your R:TW folder. It's not difficult if you know how to do a few basic computer maintenance chores.