Clone this repository into $ZSH_CUSTOM/plugins (by default ~/.oh-my-zsh/custom/plugins)

git clone https://github.com/Downager/zsh-helmfile ${ZSH_CUSTOM:-~/.oh-my-zsh/custom}/plugins/zsh-helmfile

Add the plugin to the list of plugins for Oh My Zsh to load (inside ~/.zshrc):

plugins=(zsh-helmfile)