Warning: This post is over 4 years old, so some of what you read might be out-of-date.
I'm going to presume you already have Valet and Homebrew setup. To install Imagick, there's a couple of commands you'll need to run:
brew install imagemagickbrew install pkg-config(also if it's already done)pecl install imagickvalet restart
In case you get a Connection Timeout when visiting one of your Valet sites, you may need to reinstall Valet with valet uninstall && valet install && valet park.