Catkin can be installed by simply following the CMake standard build/install process:
$ cd /path/to/catkin/src
$ mkdir build
$ cd build
$ cmake ..
$ make
$ (sudo) make install
Alternatively you can build catkin in a workspace with other catkin packages (this is sort of like a bootstrap):
$ mkdir catkin_ws
$ cd catkin_ws
$ mkdir src
$ cd src
... Checkout catkin and any other catkin based packages here...
$ cd ..
$ ./src/catkin/bin/catkin_make
$ ./src/catkin/bin/catkin_make install # this passes the 'install' target to make
Hope that gets you going.