First, install the build dependencies for amarok. Mostly, these are just headers and support libraries to hook into KDE. On debian-based systems, you can simply run # apt-get install build-essential git-core; apt-get build-dep amarok as root.
Then, using git, clone the amarok repository from KDE's anongit:
git clone git://anongit.kde.org/amarok.git; cd amarok
Before doing anything else, you should check the README and INSTALL files to see if anything has changed in these steps. They will note any important build updates.
Create a build directory so the Amarok build doesn't spew files all over the source:
mkdir build; cd build
Generate the Makefile with cmake:
cmake .. -DCMAKE_INSTALL_PREFIX=`kde4-config --prefix`
-DCMAKE_INSTALL_PREFIX tells cmake where the Makefile should place the system-installed files. On my system, $ kde4-config --prefix evaluates to /usr.
And then execute it to build the source using make:
make
Building amarok will take a bit. You may want to grab a mug of your favorite caffienated beverage.
After it's done, run (as root):
`make install`
To execute the install target and copy files into the prefix set earlier. If you don't want to install amarok globally (for example, if its already installed and you don't want to overwrite that version with the development one), you can rerun cmake with a different prefix, such as a directory under your $HOME, and run make install as a normal user.
Then (after spawning a new shell), you can simply run amarok anywhere to start it, or start it from your system's program launcher.