Customizing NEXTGEN Gallery To link images to pages

Instead of linking to lightbox I'm trying to link my thumbnail images to a page or 3rd party website.

I found http://astronautweb.co/nextgen-gallery-custom-thumbnail-link/ but was from 2011. I'm not able to follow it because I think the file structures are different now that we're all the way in 2018. I'm not seeing where I need to make an edit to the nggallery.php file in the plugin editor.

From my manage gallery page I want to add a url input field to link that thumbnail in the ALT TITLE TEXT / DESCRIPTION column.

Anyone familiar with how to do this or can direct me to a more recent tutorial? seems everything I found is very old. I'm also looking for plugins but they dont seem to have what I need.

Thanks