First you need to compile modules of course. After compiling the kernel. do "make modules" and then "INSTALL_MOD_PATH=. make modules_install"

When you're done, you will find the modules under the lib/modules/<version_number> directory on your build directory.
Take a tar of the modules directory, untar it to /lib/modules/ of your device, and say "depmod -a <version_number>" in the <version_number> directory.