git clone git://github.com/tarruda/zsh-autosuggestions ~/.zsh-autosuggestions

cat >> ~/.zshrc << "EOF"
source ~/.zsh-autosuggestions/autosuggestions.zsh

# Enable autosuggestions automatically
zle-line-init() {
	zle autosuggest-start
}
zle -N zle-line-init

# use ctrl+t to toggle autosuggestions(hopefully this wont be needed)
bindkey '^T' autosuggest-toggle

# use ctrl+f to accept suggestions
bindkey '^F' autosuggest-accept-suggested-small-word
# or
bindkey '^F' autosuggest-accept-suggested-word
EOF

I recommend binding ctrl+f to autosuggest-accept-suggested-small-word, as the zsh-history-substring-search plugin will already provide a key to accept the entire suggestion