Follow Instructions on ROCM homepage to get ROCM packages, DKMS and drivers.
Current release 1.8 as the time of writing, only works on Kernel 4.13 (per my experience).
Ubuntu 18.04 can use tools like ukuu
to drop kernel to lower version and select older kernel at GRUB boot menu.
Once ROCM is installed, add below to shell profile
export PATH="$PATH:<ROCM DIRECTORY>/bin"
export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:<ROCM DIRECTORY>/hip/include"