Add custom css for the plugin.

I want to add some external css styles so that i can transform this plugin. I want to be able to create a 3 column by 2 row widget for my footer. My problem is where and how do I add this css to the plugin such that I can use the plugin directly in other sites as well.

I am new to wordpress so I would be very glad if you could give every destails step by step.

https://wordpress.org/plugins/wp-instagram-widget/