add clarifications to README

drake_cmake_installed/README.md

@@ -20,8 +20,7 @@ Drake to download.
 
 ## Build Everything
 
-Then, after cloning this repository, run the following to
-build using CMake from the current directory
+Then run the following to build using CMake from the current directory
 (`drake-external-examples/drake_cmake_installed`):
 
 ```bash
@@ -39,6 +38,10 @@ of Drake (usually last night's build). Ignore this if that
 version is desired. Otherwise, the following are alternative
 options for which version of Drake to download.
 
+To use any of these options, instead of running `install_prereqs`,
+first run the code for whichever option you prefer below. Each option
+downloads Drake to the `$HOME` directory.
+
 1. A specific version (date-stamped)
 
 ```bash
@@ -61,6 +64,15 @@ git clone https://github.com/RobotLocomotion/drake.git
 (cd drake && mkdir build && cd build && cmake -DCMAKE_INSTALL_PREFIX=$HOME/drake -DWITH_GUROBI=ON .. && make)
 ```
 
+Then ensure you have Python installed; specifically,
+install `python3-all-dev` if on Ubuntu, and use Homebrew Python
+(not Apple's system Python) if on macOS. Finally, run *Drake's*
+`install_prereqs` script to install the remaining system packages:
+
+```bash
+$HOME/drake/share/drake/setup/install_prereqs
+```
+
 # Examples
 
 Drake-specific examples: