I presume you've built the package so now have a file called something like tvheadend-4.3.xxxx-1-armv7h.pkg.tar.xz. In which case, as root, do @pacman -U tvheadend-4.3.xxxx-1-armv7h.pkg.tar.xz@. If you've chosen the git package you can repeat the build and install process whenever you wish to keep up-to-date.
To start the application, do, as root, @systemctl start tvheadend@. To make it automatically start at boot, do @systemctl enable tvheadend@. Pacman should have created a user 'hts' which will run the application and own the config and video files.