Standard Version
To install the standard version, go to the Plugin section of your WordPress installation. Search for "Advanced Custom Fields" and install/activate. You will now have access to a new area in the WordPress administrative area labeled "Custom Fields" where you can create your first custom field group.
Pro Version
To install the pro version, download from the "Licenses and Downloads" area of the My Account section of the ACF website. Then go to the Plugin section of your WordPress installation. Upload the file, and then install/active. You will now have access to a new area in the WordPress administrative area labeled "Custom Fields" where you can create your first custom field group. To register your copy of ACF, use the license key (also available in the "Licenses and Downloads" area of your ACF account area) and paste it in the license field under the following menu area in your WordPress installation: Custom Fields > Updates. You can upgrade an existing license at any time by logging into your ACF account and following the prompts.