Make a CGAL subdirectory to place CGAL and the build directory into.
mkdir CGAL
cd CGAL
Clone cgal
git clone https://github.com/CGAL/cgal.git
cd cgal
Checkout the release we want to use (this is currently the latest stable)
git checkout releases/CGAL-4.13
cd ..
Make the build directory
mkdir build
cd build
Now we can configure it all with cmake and then make and install
cmake ../.
make
sudo make install