May I suggest simply typing m-a in a terminal, which should give you a
menu-based front-end for module-assistant. Using this you can easily go
through the steps required for building and installing the module.
Otherwise, the procedure is:
#m-a update
which checks which module packages are already on your system; and
#m-a a-i realtime-lsm
(auto-install) which ensures you have the kernel headers and other necessary
packages installed, downloads the module source if needed, then builds and
installs it. Simple as that!
One little quirk I've noticed on my system is that a freshly-built realtime
module won't load at first until I reboot. It's fine after that.
HTH,
John