The masonry plugin’s shortcode builder can be found under the WordPress Admin Dashboard > Settings > CG Masonry

The tabs Main Layout, Post Loading, Brick Header, Brick Content and Brick Footer divide the customisation setting into manageable categories

Build Shortcode button

This button generates the shortcode based on the settings you have selected, any settings that have not been changed from default are not included, only settings that have changed.

Import button

This button is effectively the reverse of the Build Shortcode button, it allows you to paste into the Textarea a previously created shortcode, when the Import button is clicked it will adjust the settings to reflect the pasted in shortcode.  More guidance is available here

Preview button

This button applies the currently selected settings to the preview brick to the right of the shortcode builder page.

Alt. Preview button

This button applies the currently selected alternate settings to the preview brick to the right of the shortcode builder page.  The alternate (Alt.) setting apply to the even numbered bricks and the primary settings applying to the odd numbered bricks.

Reset button

This button sets all setting back to the defaults.