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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