Building with AUR-helpers
For example if you use yaourt, issue:
yaourt -S tomahawk
For other AUR-helpers refer to their documentation or here: https://wiki.archlinux.org/index.php/AUR_Helpers#Comparison_Table
Follow instructions, install and/or build missing dependencies along the way.
Building by hand
Install dependencies from AUR:
libechonest-git: http://aur.archlinux.org/packages.php?ID=41904
After downloading the source packages for each of them do:
cd path/to/dir/containing/PKGBUILD
makepkg -i
Install Tomahawk:
wget http://aur.archlinux.org/packages/tomahawk/tomahawk.tar.gz
tar zxf tomahawk.tar.gz
cd tomahawk
makepkg -i
Development version
If you feel like trying some bleeding-edge Tomahawk code just replace 'tomahawk' with 'tomahawk-git' above.